Photon, Electron, and Ion Probes of Polymer Structure and Properties 9780841206397, 9780841208322, 0-8412-0639-2

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Photon, Electron, and Ion Probes of Polymer Structure and Properties
 9780841206397, 9780841208322, 0-8412-0639-2

Table of contents :
Title Page ......Page 1
Half Title Page ......Page 3
Copyright ......Page 4
ACS Symposium Series......Page 5
FOREWORD......Page 6
PdftkEmptyString......Page 0
PREFACE......Page 7
1 Resonant Electron Scattering and Anion States in Polyatomic Molecules......Page 10
Use Of ETS To Study Substituent Effects......Page 11
Comparison Of Spectra Of Gas Phase And Condensed Phase Anions......Page 12
Conclusions and Discussion......Page 14
Literature Cited......Page 17
2 Negative Ion States of Polyatomic Molecules1......Page 20
Nonaromatic Double-Bonded Structures......Page 21
Benzene and Derivatives......Page 30
Selected Halocarbons......Page 36
Literature Cited......Page 40
3 Electronic States and Excitations in Polymers......Page 44
Literature Cited......Page 52
4 Exciton States and Exciton Transport in Molecular Crystals......Page 54
Principles......Page 55
Untransformed Coupling......Page 56
Transformed Coupling......Page 58
Diffusion Constant......Page 59
Discussion......Page 60
Literature Cited......Page 63
Band Theory, Localization and Percolation......Page 66
Excitation Percolation......Page 67
Scaling Experiments......Page 69
Epilogue......Page 71
Abstract......Page 72
Literature Cited......Page 73
6 Localization of Electronic States in Polymers and Molecular Solids......Page 74
Relaxation and Fluctuations......Page 75
Localization......Page 76
Contact Charge Exchange......Page 78
Literature Cited......Page 79
Method......Page 82
Polyacetylenes and Polydiacetylenes......Page 83
Concluding Remarks......Page 87
Literature Cited......Page 88
8 Spectroscopic Studies of Polydiacetylenes......Page 90
Optical Spectroscopy......Page 93
Electron Spin Resonance Spectroscopy......Page 100
Acknowledgements......Page 107
Literature Cited......Page 108
The Nuclear Distortion Problem......Page 114
Forces Acting On The Nuclei......Page 115
The Blue Shift In Polydiacetylenes:......Page 116
Abstract......Page 118
Literature Cited......Page 119
10 Models of Radical Cation States in Molecules, Molecular Solids, and Polymers......Page 121
CNDO/S3 Model......Page 122
Relaxation In Isolated Molecules......Page 123
Relaxation In Condensed Media......Page 124
Literature Cited......Page 125
Historical Background......Page 128
Photoelectron Spectra of Molecular Solids: Basic Issues......Page 129
Some Technical Aspects of the UPS of Insulating Solids......Page 133
Examples of Contributions of Intermolecular Relaxation Effects to UPS Spectra of Molecular Solids......Page 134
Appendix: Sample Preparation and some other Experimental Details Relevant to the Study of Polystyrene and Poly(2-Vinyl Pyridine)......Page 151
Literature Cited......Page 152
Methods For The Determination of One-Electron Bands......Page 157
Relations Between One-Electron Bands And PS Spectra......Page 161
Comparisons Between Theoretical and Experimental XPS Valence Spectra......Page 163
Abstract......Page 171
Literature Cited......Page 172
13 Electronic Structure of Polymers X-Ray Photoelectron Valence Band Spectra......Page 175
Experiment......Page 176
Substitution Effects in the Valence Bands......Page 182
Isomerism......Page 194
Other Informations Available from the Valence Band Spectra......Page 202
Acknowledgements......Page 203
Literature Cited......Page 204
14 Identification of Chemical States by Spectral Features in X-Ray Photoelectron Spectroscopy......Page 208
Abstract......Page 218
Literature Cited......Page 222
15 Chemical Labels to Distinguish Surface Functional Groups Using X-Ray Photoelectron Spectroscopy(ESCA)......Page 225
Primary......Page 228
Carboxylic Acid Group Model System......Page 229
Literature Cited......Page 239
16 Interactions of Ion Beams with Organic Surfaces Studied by XPS, UPS, and SIMS......Page 240
Abstract......Page 248
Literature Cited......Page 249
17 The Modification, Degradation, and Synthesis of Polymer Surfaces Studied by ESCA......Page 250
Migration Phenomena (9)......Page 258
The Plasma Synthesis Of Ultra Thin Fluoropolymer Films......Page 262
An ESCA Investigation Of The Surface Chemistry Of The Nitration And Denitration Of Cellulose Materials......Page 272
Literature Cited......Page 292
Absolute and Relative Signal Intensities......Page 295
Absolute and Relative Binding Energies......Page 304
Conclusion......Page 317
Literature Cited......Page 318
EXPERIMENTAL......Page 320
A) Regular Block Copolymers.......Page 322
B) Random Block Copolymers......Page 332
PEO/PS Layers......Page 334
Conclusions......Page 336
Literature Cited......Page 338
20 Metal-Polymer Interfaces: Studies with X-Ray Photoemission......Page 340
Results......Page 341
Discussion......Page 345
Literature Cited......Page 350
21 Surface Characterization of Plasma-Fluorinated Polymers......Page 353
Results and Discussion......Page 354
Fluorination of Other Polymers......Page 361
Treatment Of Polymer Powders......Page 363
Abstract......Page 368
Literature Cited......Page 369
22 ESCA and SEM Studies on Polyurethanes for Biomedical Applications......Page 371
Results and Discussion......Page 372
Acknowledgement......Page 381
Literature Cited......Page 382
23 Surface Analysis of Silicon: Alloyed and Unalloyed LTI Pyrolytic Carbon......Page 383
Background......Page 384
Materials and Methods......Page 385
Results and Discussion......Page 388
Abstract......Page 402
Literature Cited......Page 403
24 Oxidation of Polystyrene and Pyrolytic Carbon Surfaces by Radiofrequency Glow Discharge......Page 405
Experimental......Page 407
Results and Discussion......Page 411
Abstract......Page 416
Literature Cited......Page 417
25 ESCA Studies of Polyimide and Modified Polyimide Surfaces......Page 419
Experimental......Page 420
Results and Discussion......Page 421
Literature Cited......Page 432
C ......Page 434
F ......Page 435
N ......Page 436
P ......Page 437
S ......Page 438
X ......Page 439

Citation preview

Photon, Electron, and Ion Probes of Polymer Structure and Properties

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Photon, Electron, and Ion Probes of Polymer Structure and Properties EDITOR

David W. Dwight,

Virginia Polytechnic Institute and State University Thomas J. Fabish,

EDITOR

Ashland Chemical Company H. Ronald Thomas,

EDITOR

Xerox Corporation

Based on a symposium cosponsored by the Divisions of Organic Coatings and Plastics and Polymer Chemistry at the 179th Meeting of the American Chemical Society, Houston, Texas, March 25-27, 1980.

ACS

SYMPOSIUM AMERICAN

CHEMICAL

W A S H I N G T O N , D. C .

SERIES

162

SOCIETY 1981

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Library of Congress CIP Data Photon, electron, and ion probes of polymer structure and properties. (ACS symposium series; 162, ISSN 0097-6156) "Based on a symposium co-sponsored by the Divisions of Organic Coatings and Plastic d Polyme Chemistry at the 179th meetin ical Society, Houston, Texas, Marc Includes bibliography and index. 1. Polymers and polymerization—Analysis—Congresses. 2. Photoelectron spectroscopy—Congresses. I. Dwight, David W., 1941. II. Thomas, H . Ronald, 1942. III. Fabish, Thomas J., 1938- . IV. American Chemical Society. Division of Organic Coatings and Plastics Chemistry. V. American Chemical Society. Division of Polymer Chemistry. VI. Series: ACS symposium series; 162. QD380.P53 ISBN 0-8412-0639-2

547.7'046

81-10816 AACR2 ASCMC 8 162 1-442 1981

Copyright © 1981 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each article in this volume indicates the copyright owner's consent that reprographic copies of the article may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating new collective work, for resale, or for information storage and retrieval systems. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of anyrightor permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. PRINTED IN THE UNITED STATES OF AMERICA

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

ACS Symposium Series M . Joa

Advisory Board David L. Allara

James P. Lodge

Kenneth B. Bischoff

Marvin Margoshes

Donald D. Dollberg

Leon Petrakis

Robert E. Feeney

Theodore Provder

Jack Halpern

F. Sherwood Rowland

Brian M . Harney

Dennis Schuetzle

W. Jeffrey Howe

Davis L. Temple, Jr.

James D. Idol, Jr.

Gunter Zweig

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

FOREWORD The ACS SYMPOSIU

a medium for publishing symposi quickly format of the Series parallels that of the continuing ADVANCES IN CHEMISTRY SERIES except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. Both reviews and reports of research are acceptable since symposia may embrace both types of presentation.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PREFACE

T

he last decade witnessed a dramatic growth in the use of energetic beam techniques to elucidate the electronic structures of atoms and molecules. Photon, electron, and ion spectroscopies applied to solids gave birth to a new level of surface sensitivity for studies of chemical structure and bonding. The time was right to provide a benchmark for the state of current knowledge and futur possibilitie i th field. Thefirstobjective o was to focus attention on the experimental and theoretical techniques currently being used to describe anion and cation states in large molecules and polymeric solids, with special emphasis being placed on the consequences to electronic structure incurred upon condensation from the gaseous into the solid state. It is, therefore, appropriate to begin with a review of the state of knowledge of anion states in large, isolated molecules as revealed by elastic and inelastic electron scattering, electron swarm, mass spectrographic, and ion-cyclotron resonance experiments. Insight into the effects of condensation on the nature of the lowest energy excitations that govern many of the chemical and physical properties of polymers is provided by results from high-energy electron energy loss spectroscopic studies on representative saturated polymers. Energy and charge transport in saturated and conjugated polymeric solids represent limiting cases in the applicability of the precepts of band theoretical descriptions of the electronic structure of solids. Discussions of the nature of intrinsic localized electronic states and their consequences to treatments of transport phenomena in such materials comprise an important section of these proceedings. Experimental studies of fundamental excitations in conjugated polymers are interpreted within the framework of current theoretical electronic structural calculations and physical structure characterizations. The instabilities peculiar to this class of materials that are responsible for their departure from metallic behavior are identified explicitly. Considering the valence levels, the synergistic effect of combining spectroscopic measurements with theoretical calculations is illustrated by two pairs of chapters: (1) ultraviolet photoemission and optical absorption data compared to a spectroscopically parameterized CNDO/S3 model, and (2) x-ray photoemission compared to ab initio and intermediate approximation MO calculations. ix In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

The firm theoretical understanding of the spectra of model molecules has been a major impetus to apply the techniques to complex, unknown solid-state structures of technological importance. Characterization of atomic composition, structure, and bonding in the surface and subsurface of "practical" specimens is thefirststep. Most studies interpret changes in the core-level spectra in terms of surface-chemical mechanisms involved in, for example, processing conditions, exposure to inert or reactive gas plasmas, chemical reactions, or aggressive service environments. Five chapters treat the basics as well as new developments in analytical methodology, emphasizing x-ray photoelectron spectroscopy for the identification of structure and bonding in polymer surfaces of increasing complexity. Each of the seven important levels of information available in the core-level data is illustrated in detail with special emphasis upon: (1) Auger peak positions kinetic energy dependence of peak intensities for information on compositional variation with depth; and (3) enhanced resolution of functional groups with chemical derivatives. Explicitly developed are models of several theoretical multiphase distributions, with corresponding depth-profile results on thin-film plasma polymers, phase-separated block copolymers, and chemical reactions on fiber surfaces. Ion impact is treated from three points of view: as an analyticalfingerprinttool for polymer surface analysis via secondary ion mass spectroscopy, by forming unique thinfilmsby introducing monomers into the plasma, and as a technique to modify polymer surface chemistry. New experimental results on specific polymer material problems are presented in the last nine chapters. Several cases involve the study of polymers from commercial sources. The topics include: (1) surface chemistry as induced by (a) outdoor weathering, (b) chemical reactions, and (c) plasma exposure; (2) chemical bond formation at the polymer -metal interface; and (3)biomaterials characterization and relationship to blood compatibility. In summary, these proceedings provide a survey of the fundamentals and applications of photon, electron, and ion probes to polymers up to 1980. The contributors include many pioneers—from the basic studies performed with specialized, custom apparatus on small, gas-phase molecules to those utilizing standard commercial spectrometers to characterize practical polymer systems. We hope this work will contribute to the attainment of the ultimate objective of a unified description of the electronic structures and properties of polymeric solids. Acknowledgment is made to the donors of the Petroleum Research Fund (PRF #12290-SEO), administered by the American Chemical Society, for support in the way of travel expenses of the seven invited foreign speakers. Also Dr. Dwight gratefully acknowledges the support of x In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

the Polymer Program of the Materials Division, National Science Foundation (Grant DMR 78-05429) and the Army Research Office (Grant DAAG29-80-K-0093) for partial support during the organization of the meeting and the preparation of the manuscript. Drs. Fabish and Thomas gratefully acknowledge support from Ashland Chemical Company and Xerox Webster Research Center, respectively. The editors are indebted to Kathy Fuller for typing and organizing the manuscripts in this book. DAVID W. DWIGHT

Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061 THOMAS J . FABISH

Ashland Chemical Company Columbus, Ohio 43216 H . RONALD THOMAS

Xerox Webster Research Center Rochester, New York 14644 March 27, 1981

xi In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

1 Resonant Electron Scattering and Anion States in Polyatomic Molecules K E N N E T H D. JORDAN Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260 PAUL D. BURROW Department of Physics, University of Nebraska, Lincoln, N E 68588

P h o t o e l e c t r o n s p e c t r o s c o p y ( P E S ) (1, 2) and more r e c e n t l y e l e c t r o n t r a n s m i s s i o n s p e c t r o s c o p y ( E T S ) (3, 4) h a v e p r o v i d e d much i n f o r m a t i o n on t h e c a t i o n and a n i o n s t a t e s , r e s p e c t i v e l y , o f many h y d r o c a r b o n s . W i t h i n t h e c o n t e x t o f t h e Koopmans' Theorem (KT) a p p r o x i m a t i o n , t h e c a t i o n s t a t e s c a n o f t e n be a s s o c i a t e d w i t h t h e f i l l e d o r b i t a l s and t h e a n i o n s t a t e s w i t h t h e u n f i l l e d orbitals of a molecule. I n t h i s s e n s e t h e s e two methods a r e complementary. However t h e r e a r e i m p o r t a n t d i s t i n c t i o n s b e t w e e n t h e s e two s p e c t r o s c o p i c methods w h i c h a r i s e i n p a r t from t h e v e r y d i f f e r e n t l i f e t i m e s o f t h e a n i o n s and c a t i o n s . I n p h o t o e l e c t r o n s p e c t r o s c o p y one a n a l y z e s t h e e n e r g y o f electrons photoejected from a m o l e c u l e ( o r a t o m ) . The c a t i o n states associated with simple one-electron i o n i z a t i o n s are u s u a l l y s t a b l e or long l i v e d . Hence t h e l i n e w i d t h s i n p h o t o e l e c t r o n s p e c t r a a r e g e n e r a l l y d e t e r m i n e d by t h e e x p e r i m e n t a l r e s o l u t i o n , t y p i c a l l y 0.02 eV f o r c o m m e r c i a l s p e c t r o m e t e r s , o r by t h e d e n s i t y of v i b r a t i o n a l or r o t a t i o n a l s t a t e s i n l a r g e m o l e c u l e s . The a u t o i o n i z a t i o n t i m e s _ a ^ s o c i a t e d w i t h c e r t a i n s h a k e - u p s t a t e s may be much l e s s t h a n 10 s e c , l e a d i n g i n some c a s e s t o s p e c t r a l w i d t h s g r e a t e r than the e x p e r i m e n t a l r e s o l u t i o n . I n e l e c t r o n s c a t t e r i n g m e t h o d s , s u c h as E T S , one l o o k s f o r the r a p i d v a r i a t i o n s i n the s c a t t e r i n g c r o s s s e c t i o n a s s o c i a t e d w i t h t h e t e m p o r a r y c a p t u r e o f an e l e c t r o n by a m o l e c u l e . I n t h e s e methods o n l y a n i o n s t a t e s w h i c h l i e above t h e g r o u n d s t a t e o f t h e n e u t r a l species are a c c e s s i b l e . F o r many p r o t o t y p i c a l m o l e c u l e s , (4) i n c l u d i n g b e n z e n e , b u t a d i e n e , f o r m a l d e h y d e , n a p h t h a l e n e , and styrene, a l l a n i o n s t a t e s l i e above t h e g r o u n d s t a t e o f t h e n e u t r a l m o l e c u l e , b u t f o r o t h e r s s u c h as a n t h r a c e n e , h e x a t r i e n e , and g l y o x a l t h e g r o u n d s t a t e a n i o n s l i e e n e r g e t i c a l l y b e l o w t h e g r o u n d s t a t e o f t h e n e u t r a l m o l e c u l e and c a n n o t b e s t u d i e d b y E T S . F o r TCNQ t h e g r o u n d and a t l e a s t t h e f i r s t e x c i t e d s t a t e o f t h e a n i o n a r e bound (_5) and t h e r e f o r e inaccessible t o s t u d y by e l e c t r o n s c a t t e r i n g methods. I n contrast, a l l c a t i o n states are i n p r i n c i p l e a c c e s s i b l e i n PES. 0097-6156/81/0162-0001 $05.00/0 © 1981 American Chemical Society In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

PHOTON, ELECTRON, AND ION PROBES

A s e c o n d m a j o r d i f f e r e n c e between ETS and PES r e s u l t s from the unique p r o p e r t i e s o f temporary a n i o n s t a t e s . The t e m p o r a r y a n i o n s t a t e s a s s o c i a t e d w i t h the occupaticy^ o f u n f i l e d o r b i t a l s o f most h y d r o c a r b o n s have l i f e t i m e s o f 10 - 10 s. S i n c e the FWHM r e s o l u t i o n a t t a i n a b l e i n ETS i s 0 . 0 2 - 0 . 0 5 e V , t h i s means t h a t t h e l i n e w i d t h s due t o a u t o d e t a c h m e n t a r e g e n e r a l l y g r e a t e r than the e x p e r i m e n t a l r e s o l u t i o n . I t i s p r e c i s e l y these short l i f e t i m e s w h i c h make the s t u d y o f t h e s e a n i o n s i n t h e gas p h a s e e x c e e d i n g l y d i f f i c u l t by o p t i c a l s p e c t r o s c o p y and w h i c h makes e l e c t r o n s c a t t e r i n g techniques i d e a l l y s u i t e d . E l e c t r o n t r a n s m i s s i o n s p e c t r o s c o p y can p r o v i d e i n f o r m a t i o n w h i c h i s n o t r e a d i l y a v a i l a b l e o r a c c e s s i b l e by o t h e r t e c h n i q u e s . Here we w i l l g i v e j u s t a few i l l u s t r a t i v e e x a m p l e s . A more c o m p l e t e d i s c u s s i o n o f t h e a p p l i c a t i o n s o f ETS c a n be found i n a recent review (4). Use

Of ETS To S t u d y S u b s t i t u e n

A f r e q u e n t l y encountered problem i n c h e m i s t r y concerns the e f f e c t o f a s u b s t i t u e n t on t h e p r o p e r t i e s o f a p a r e n t m o l e c u l e . P e r h a p s t h e most i m p o r t a n t , and c e r t a i n l y the most s t u d i e d (6), c l a s s o f s u b s t i t u e n t s are a l k y l groups. The h i g h e s t - l y i n g o c c u p i e d TT o r b i t a l s o f m o l e c u l e s s u c h as e t h y l e n e , f o r m a l d e h y d e , and b e n z e n e a r e w e l l known t o be d e s t a b i l i z e d by a l k y l s u b s t i t u t i o n . The s i t u a t i o n f o r the IT* o r b i t a l s i s much l e s s c l e a r . From o u r ETS s t u d i e s (4, 8) we have found t h a t a l k y l substitution d e s t a b i l i z e s t h e LUMO o f e t h y l e n e and f o r m a l d e h y d e b u t s l i g h t l y s t a b i l i z e s t h a t o f b e n z e n e , t o l u e n e , and t - b u t y l b e n z e n e . This r e s u l t i s p a r t i c u l a r l y s i g n i f i c a n t because the c o n v e n t i o n a l p i c t u r e o f r e s o n a n c e and i n d u c t i v e e f f e c t s l e a d s one t o e x p e c t t h a t a l k y l s u b s t i t u t i o n s h o u l d a l s o d e s t a b i l i z e the b e n z e n e Tr* o r b i t a l s (see F i g u r e 1 ) . Another i n t e r e s t i n g observation i s that a l k y l s u b s t i t u t i o n causes a s u b s t a n t i a l d e c r e a s e i n the l i f e t i m e o f benzene a n i o n s ( TT* s i n g l e t and t r i p l e t t r a n s i t i o n s o f the f l u o r e t h y l e n e s ( K ) ) . F o r the t r i p l e t t r a n s i t i o n s the r e s u l t eV. There i s c o n s i d e r a b l singlets. H o w e v e r , i f f l u o r o e t h y l e n e , 1 , 1 - d i f l u o r o e t h y l e n e , and t r i f l u o r o e t h y l e n e are e l i m i n a t e d , the s i n g l e t t r a n s i t i o n s o f the o t h e r f o u r s p e c i e s c a n a g a i n be w e l l r e p r e s e n t e d by a s t r a i g h t line. The s i n g l e t transitions f o r the three asymmetrically s u b s t i t u t e d molecules f a l l w e l l below t h i s l i n e . We b e l i e v e t h a t the " d e v i a t i o n " o f the s i n g l e t t r a n s i t i o n s o f these t h r e e m o l e c u l e s c a n be u n d e r s t o o d i n terms o f a s i m p l e c o n f i g u r a t i o n m i x i n g model. O n l y f o r t h ^ s e t h r e e ^ m o l e c u l e s c a n t h e TT ^ TT* c o n f i g u r a t i o n m i x w i t h t h e TT -* (TT*) configuration. This configuration m i x i n g c a u s e s an e n e r g y l o w e r i n g o f t h e i r -> TT * c o n f i g u r a t i o n . C o m p a r i s o n Of S p e c t r a O f Gas Phase And Condensed P h a s e

Anions

I n a s o l v e n t s u c h as m e t h y l t e t r a h y d r o f u r a n (MTHF), a n i o n and c a t i o n s t a t e s o f an u n s a t u r a t e d hydrocarbon are typically s t a b i l i z e d by 1 - 1 . 5 e V . F o r e x a m p l e , t h e g r o u n d s t a t e a n i o n s o f b u t a d i e n e , b e n z e n e , n a p h t h a l e n e , and s t y r e n e , w h i c h a r e u n s t a b l e b y l . l eV o r l e s s i n t h e gas p h a s e , a r e a l l s t a b l e i n MTHF g l a s s a t 77 K m a k i n g p o s s i b l e t h e i r s t u d y by o p t i c a l s p e c t r o s c o p y . On t h e o t h e r h a n d , t h e a n i o n s o f e t h y l e n e and 1,4 c y c l o h e x a d i e n e w h i c h a r e u n s t a b l e by a b o u t 1.8 eV i n t h e gas phase h a v e n o t b e e n p r e p a r e d i n MTHF g l a s s . P r e s u m a b l y , t h e u s e o f a more p o l a r s o l v e n t would a l l o w the p r e p a r a t i o n o f these a n i o n s i n a g l a s s . To i l l u s t r a t e t h e e f f e c t o f s o l v a t i o n on t e m p o r a r y a n i o n s we w i l l c o n s i d e r the naphthalene m o l e c u l e . T h i s m o l e c u l e i s p a r t i c u l a r l y i n t e r e s t i n g b e c a u s e i t i s an a l t e r n a n t h y d r o c a r b o n ( 1 4 ) , and f o r s u c h m o l e c u l e s , t h e p a i r i n g t h e o r e m ( L 5 ) p r e d i c t s t h a t t h e a n i o n and c a t i o n s p e c t r a s h o u l d be i d e n t i c a l . T h i s theorem i s v a l i d f o r b o t h H u c k e l and PPP m o d e l H a m i l t o n i a n s , b u t i s n o t v a l i d f o r ab i n i t i o o r CNDO c a l c u l a t i o n s . I t has b e e n found ( 1 0 ) t o be t r u e t o a good a p p r o x i m a t i o n ( ^ 0 . 1 eV) i n o r g a n i c g l a s s e s ( 1 6 ) . The ETS s p e c t r a a l l o w s an e x a m i n a t i o n o f the v a l i d i t y o f t h i s Q

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4

PHOTON, ELECTRON, AND ION PROBES

T

12.4

'

1

1

'

1

"

1

12.8 ' 13.2 I P - E A (eV)

1 r

'

13.6

Chemical Physics Letters

Figure 2.

Vertical singlet and triplet ir - > t t * excitation energies of ethylene and thefluoroethylenesas a function of (IP-EA) (10)

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

1.

JORDAN AND BURROW

Electron Scattering and Anion States

t h e o r e m f o r gas phase s p e c i e s . U s i n g d i f f e r e n c e s b e t w e e n gas p h a s e E A ' s , we c o n s t r u c t (17) gas p h a s e t r a n s i t i o n e n e r g i e s f o r c o m p a r i s o n w i t h the c o n d e n s e d phase v a l u e s . The ETS and PES s p e c t r a a r e p r e s e n t e d i n F i g u r e 3 i n s u c h a way as t o f a c i l i t a t e c o m p a r i s o n o f t h e s p a c i n g s b e t w e e n t h e a n i o n and c a t i o n energy l e v e l s . I n F i g u r e 4 we p r e s e n t t h e a n i o n e x c i t a t i o n e n e r g i e s o b t a i n e d from E T S , from a b s o r p t i o n s t u d i e s i n MTHF g l a s s , s e m i - e m p i r i c a l PPP and L o n g u e t - H i g g i n s - P o p l e C I c a l c u l a t i o n s , and f i n a l l y from P E S , a s s u m i n g t h e v a l i d i t y o f t h e p a i r i n g theorem. P e r h a p s t h e most s t r i k i n g f e a t u r e t o be g l e a n e d from t h i s f i g u r e i s t h a t f o r the l o w - l y i n g t r a n s i t i o n s the e n e r g i e s d e t e r m i n e d by a l l methods a r e i n good a g r e e m e n t , w h i l e f o r t h e h i g h e r l y i n g t r a n s i t i o n s t h e gas phase r e s u l t s l i e c o n s i d e r a b l y above the o t h e r s . We h a v e i n t e r p r e t e d t h i s i n terms o f a n i o n l i f e t i m e s , noting that, i n general t h e l i f e t i m e d e c r e a s e s as one goes t o increasingly high-lying s t a t e s n e a r 5 eV i n n a p h t h a l e n These t i m e s a r e o n l y a f a c t o r o f 2 - 3 g r e a t e r t h a n t h e t i m e f o r an e l e c t r o n w i t h 5 eV o f k i n e t i c e n e r g y t o t r a v e l a d i s t a n c e e q u a l to the " s i z e " o f the n a p h t h a l e n e m o l e c u l e . Hence t h e wavefunctions a s s o c i a t e d w i t h these very s h o r t - l i v e d resonances h a v e much l e s s l o c a l i z a t i o n o f c h a r g e i n t h e m o l e c u l a r r e g i o n t h a n do t h o s e a s s o c i a t e d w i t h l o n g - l i v e d r e s o n a n c e s . As a r e s u l t , t h e s h o r t - l i v e d resonances undergo l e s s e l e c t r o n i c r e l a x a t i o n o f the charge d e n s i t y a s s o c i a t e d w i t h the n e u t r a l m o l e c u l e . I n t h e c o n d e n s e d p h a s e , where t h e l i f e t i m e s o f t h e a n i o n s a r e i n c r e a s e d and t h e i r w a v e f u n c t i o n s s h o u l d be c o n s i d e r a b l y more l o c a l i z e d , a l l the a n i o n s t a t e s s h o u l d e x p e r i e n c e c o n s i d e r a b l e reorganization. I n o t h e r w o r d s , when compared t o t h e c o n d e n s e d p h a s e a n i o n s , t h e h i g h - l y i n g gas p h a s e a n i o n s a r e " n o n - r e l a x e d " and t h e l o w - l y i n g a n i o n s e s s e n t i a l l y " f u l l y r e l a x e d " . Clearly, any a n i o n s t a t e which l i v e s s u f f i c i e n t l y l o n g to d i s p l a y s t r u c t u r e due t o n u c l e a r m o t i o n w i l l be e s s e n t i a l l y f u l l y relaxed electronically. I t has b e e n n o t e d by Duke and c o w o r k e r s ( 1 8 ) t h a t t h e v a r i o u s c a t i o n s t a t e s o f unsaturated hydrocarbons are s t a b i l i z e d to n e a r l y the same e x t e n t g o i n g f r o m t h e gas phase t o the c o n d e n s e d phase. T h e r e i s b e t t e r p a i r i n g b e t w e e n t h e a n i o n and c a t i o n s t a t e s i n MTHF g l a s s t h a n f o r t h e gas phase s p e c i e s . This is i n agreement w i t h t h e o b s e r v a t i o n t h a t t h e v a r i o u s a n i o n s t a t e s a r e s t a b i l i z e d by d i f f e r i n g amounts g o i n g from the gas t o t h e c o n densed phase. C o n c l u s i o n s and

Discussion

I n t h i s p a p e r we have t r i e d t o g i v e t h e r e a d e r a g e n e r a l o v e r v i e w o f t h e t y p e s o f c h e m i c a l i n f o r m a t i o n t h a t c a n be o b t a i n e d from e l e c t r o n t r a n s m i s s i o n s p e c t r o s c o p y . ETS a p p e a r s t o be i d e a l l y s u i t e d f o r s t u d y i n g t e m p o r a r y a n i o n s formed by t h e c a p -

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

5

6

PHOTON, ELECTRON, AND ION PROBES

Electron

a -o

Figure 3. Photoelectron and electron transmission spectra of naphthalene: (a) derivation of the transmitted current as a function of electron energy in naphthalene; (b) Hel spectra of naphthalene (21)

0 I

1

2 1

Ionization

Energy

1

4 1

(eV)

1

6 r

Potential (eV) Academic Press

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

JORDAN AND BURROW

Electron Scattering and Anion States

Chemical Physics

Figure 4. Anion excitation energies in eV for naphthalene. (ETS) Energies derived from electron transmission measurements in the gas phase; (Soln) optical absorption studies in anions in MTHF glass; (PPI, CI) theoretical energies; (PT) values derived from the cation spectrum by applications of the Pairing Theorem W).

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTON, ELECTRON, AND ION PROBES

8

t u r e o f e l e c t r o n s i n t o l o w - l y i n g u n f i l l e d TT* o r b i t a l s . Temporary a n i o n s a s s o c i a t e d w i t h a* o r b i t a l s a p p a r e n t l y are too s h o r t - l i v e d i n g e n e r a l t o be d i s c e r n a b l e i n the t o t a l s c a t t e r i n g crosssection. The o n l y s a t u r a t e d h y d r o c a r b o n f o r w h i c h we h a v e o b s e r v e d a shape r e s o n a n c e i s c y c l o p r o p a n e (19). However, t h i s p r o b a b l y s h o u l d n o t be c o n s i d e r e d an e x c e p t i o n s i n c e some o f t h e s i g m a o r b i t a l s o f t h i s m o l e c u l e a r e known t o have a p p r e c i a b l e p i character. F o r v e r y s h o r t - l i v e d a n i o n s t a t e s , the c r o s s s e c t i o n s f o r v i b r a t i o n a l e x c i t a t i o n w i l l p r o v i d e a more s e n s i t i v e means o f detection. F o r example, W a l k e r , et a l . (20) have l o c a t e d a a * r e s o n a n c e i n e t h y l e n e between 3 and 6 e V . Ack now1edg ement s T h i s r e s e a r c h has b e e n s u p p o r t e d by t h e P e t r o l e u m R e s e a r c h F u n d , a d m i n i s t e r e d by t h e A m e r i c a n C h e m i c a l S o c i e t y and by R e search C o r p o r a t i o n .

Abstract The temporary anions associated with low-lying u n f i l e d orbitals generally have lifetimes in the range of 10 - 10 s in the gas phase. This paper discusses the use of electron transmission spectroscopy (ETS) to provide information on these short-lived anions. The sensitivity of the lifetimes to changes in symmetry is illustrated by comparing the electron transmission of benzene and various alkyl substituted benzenes. Knowledge of the gas phase electron affinities for a series of molecules, such as the fluoroethylenes, provides new insight into substituent effects. The correlation between the trends in the singlet and triplet excitation energies of the neutral molecules and the quantity (IP - EA) is also examined. The energy levels of the anion states of naphthalene and styrene as determined from the gas phase studies are compared to those obtained from optical absorption measurements on the anions in organic matrices as well as with theoretical predictions. It is demonstrated that the effect of solvation on the energy of an anion state depends on its lifetime, with larger shifts being observed for the shorter lived anions. -12

-15

Literature Cited 1.

Turner, D.W., Baker, C., Baker, A.D. and Brundle, C.R., "Molecular Photoelectron Spectroscopy" (John Wiley and Sons, New York, 1970).

2.

Rabalais, J.W., "Principles of Ultraviolet Photoelectron Spectroscopy", (John Wiley and Sons, New York, 1977).

3.

Sanche, L. and Schulz, G.J., J. Chem. Phys., 1973, 58, 479.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Electron Scattering and Anion States

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Jordan, K.D. and Burrow, P.D., Accts. of Chem. Res., 1978, 11, 341.

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Klots, C.E., Compton, R.N., Raaen, V.F., J. Chem. Phys., 1974, 60, 1177.

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Libit, L. and Hoffman, R., J. Amer. Chem. Soc., 1974, 96, 1370; Mosclet, P., Grosjean, D., Mouvier, G. and Dubois, J . , J. Electron Spectry. and Related Phenom., 1973, 2, 225.

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Chiu, N.S., Johnston, A.D., Burrow, P.D. and Jordan, K.D., to be submitted to J. Electron Spectry. and Related Phenom.

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Jordan, K.D., Burrow, P.D. and Michejda, J.A., J. Am. Chem. Soc., 1976, 98, 1245.

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Chiu, N.S., Burrow J. Chem. Phys.

10. Chiu, N.S., Burrow, P.D. and Jordan, K.D., Chem. Phys. Letters, 1979, 68, 121. 11. Brundle, C.R., Robin, M.B., Kuebler, N.A. and Basch, H., J. Am. Chem. Soc., 1972, 94, 1451. 12. See, for example, Michl, J. and Becker, R.S., J. Chem. Phys., 1967, 46, 3889. 13. Jordan, K.D., Ph.D. Thesis (MIT, 1974), unpublished. 14. A system is defined as alternant if one can identify alternate positions of the skeleton (as with an asterick) and have no two adjacent positions both "starred" or "unstarred". 15. L. Salem, "The Molecular Orbital Theory of Conjugated Systems" (W.A. Benjamin, New York, 1966). 16. T. Shida and S. Iwata, J. Am. Chem. Soc., 1973, 95, 3473; Hoijtink, G.J., Velthorst, N.H. and Zandstra, P.J., Mol. Phys., 1960, 3, 533. 17. Jordan, K.D. and Burrow, P.D., Chem. Phys., 1980, 45, 171. 18. Duke, C.B., Salaneck, W.R., Fabish, T.J., Ritsko, J.J., Thomas, H.R. and Paton, A., Phys. Rev. B., 1978, 18, 5717. 19. Burrow, P.D. and Jordan, K.D., unpublished results. 20. Walker, I.C., A. Stamatovic, A. and Wong, S.F., J. Chem. Phys., 1978, 69, 5532.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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21. Heilbronner, E. and Maier, J.P., "Electron Spectroscopy: Theory, Techniques and Applications, Vol. I", ed. C.R. Brundle and A.D. Baker (Academic Press, Inc., New York, 1977) p. 228. RECEIVED December 22,

1980.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2 1

Negative Ion States of Polyatomic Molecules L. G. CHRISTOPHOROU Atomic, Molecular and High Voltage Physics Group, Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, T N 37830 and Department of Physics, University of Tennessee, Knoxville, T N 37916

I n t h i s p a p e r I s h a l l e l a b o r a t e on r e c e n t k n o w l e d g e c o n c e r n i n g ( i ) t h e number and e n e r g i e s , ( i i ) t h e c r o s s s e c t i o n s and d e c o m p o s i t i o n s , and ( i i i ) t h e l i f e t i m e s o f n e g a t i v e i o n s t a t e s (NISs) o f polyatomic organic molecules. These a r e formed by c a p t u r e o f an e l e c t r o n i n e i t h e r t h e f i e l d o f t h e g r o u n d o r t h e f i e l d o f an e x c i t e d e l e c t r o n i c s t a t e t h r o u g h a number o f m e c h a nisms, ( ! ) and a r e m e t a s t a b l e w i t h l i f e t i m e s r a n g i n g from ^ 1 0 to >10 sec. I s h a l l r e s t r i c t m y s e l f o n l y t o two c a s e s , n a m e l y , t h o s e N I S s w h i c h a r e formed i n t h e f i e l d o f t h e g r o u n d e l e c t r o n i c s t a t e v i a a shape r e s o n a n c e and v i a a n u c l e a r - e x c i t e d F e s h b a c h r e s o n a n c e m e c h a n i s m . These two mechanisms a r e i l l u s t r a t e d i n F i g . 1. I n the former type the e l e c t r o n i s trapped i n the a t t r a c t i v e r e g i o n o f t h e p o t e n t i a l , w h i c h r e s u l t s from t h e c o m b i n e d e f f e c t o f t h e l o n g - r a n g e p o l a r i z a t i o n p o t e n t i a l and t h e s h o r t - r a n g e r e p u l s i o n due t o t h e c e n t r i f u g a l m o t i o n o f t h e two p a r t i c l e s , and t h e NIS l i e s above t h e c o r r e s p o n d i n g p a r e n t n e u t r a l s t a t e [ i . e . , i t s e l e c t r o n a f f i n i t y , E A , i s n e g a t i v e (10 sec. Their cross

1

Research sponsored by the Office of Health and Environmental Research of the US Department of Energy under contract W-7405eng-26 with Union Carbide Corporation. 0097-6156/81 /0162-0011 $06.00/0 © 1981 American Chemical Society In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTON, ELECTRON, AND ION PROBES

12

s e c t i o n s a r e l a r g e ( o f t e n >10 cm ) and as a r u l e r e a c h t h e i r maximum v a l u e a t t h e r m a l e n e r g i e s (2^). We u s u a l l y l e a r n o f t h e p r o p e r t i e s o f N I S s t h r o u g h t h e i r decay c h a n n e l s . Thus k n o w l e d g e on ( i ) i s n o r m a l l y o b t a i n e d e i t h e r from t h e e n e r g y dependence o f t h e t o t a l e l e c t r o n s c a t t e r i n g c r o s s s e c t i o n , or the c r o s s s e c t i o n f o r e l a s t i c s c a t t e r i n g , or the c r o s s section for i n e l a s t i c s c a t t e r i n g i n v o l v i n g v i b r a t i o n a l and/or e l e c t r o n i c e x c i t a t i o n — a t an a n g l e o r i n t h e f o r w a r d d i r e c t i o n — v i a the decay o f the NIS by e l e c t r o n e m i s s i o n ; t h e s e cross s e c t i o n s a r e m e a s u r e d c o n v e n i e n t l y u s i n g e l e c t r o n s c a t t e r i n g and electron transmission techniques. K n o w l e d g e on ( i ) c a n be a c q u i r e d a l s o from s t u d i e s o f t h e e n e r g y dependence o f t h e c r o s s sections f o r s p e c i f i c d i s s o c i a t i v e a t t a c h m e n t fragment ions, t o t a l d i s s o c i a t i v e attachment cross s e c t i o n s or cross s e c t i o n s f o r p a r e n t n e g a t i v e i o n f o r m a t i o n ; the l a t t e r c r o s s s e c t i o n s are u s u a l l y d e t e r m i n e d ( e s p e c i a l l y a t l o w e n e r g i e s ) by t h e swarm and t h e swarm-beam t e c h n i q u e s e l e c t r o n beam, mass s p e c t r o m e t r i method b e i n g most a p p r o p r i a t e f o r low e n e r g i e s and f o r n u c l e a r e x c i t e d Feshbach resonances. Our k n o w l e d g e on ( i i i ) come^ from e l e c t r o n s c a t t e r i n g methods when EA < 0 eV and T ^10 sec, from h i g h p r e s s u r e ^ l e c t r o n swarm s t u d i e s when EA > 0 eV and T i n t h e r a n g e 10 - 1 0 s e c , from t i m e - o f - f l i g h t mass s p e c t r o m e f r i c s t u d i e s when EA > 0 eV and ^ 10 s e c , and from i o n - c y c l o t r o n r e s o n a n c e t e c h n i q u e s when EA > 0 eV and ^10 sec ( s e e R e f . 2). T

a

M o l e c u l a r n e g a t i v e i o n s t a t e s are abundant; t h e i r e n e r g i e s , c r o s s s e c t i o n s , and l i f e t i m e s a r e s t r o n g l y a f f e c t e d by t h e d e t a i l s o f the m o l e c u l a r s t r u c t u r e as w e l l as t h e medium w h i c h s u r r o u n d s the m e t a s t a b l e a n i o n s . The c r o s s s e c t i o n s and l i f e t i m e s o f the N I S s a r e f u n c t i o n s o f t h e e l e c t r o n e n e r g y e. O f t e n t h e NISs a r e d e s c r i b e d — t h e i r e n e r g i e s a p p r o x i m a t e d and t h e i r numbers r a t i o n a l i z e d — i n terms o f the u n o c c u p i e d m o l e c u l a r o r b i t a l s o f t h e n e u t r a l m o l e c u l e . These a s p e c t s o f NISs w i l l be a p p a r e n t from t h e s e l e c t e d d a t a p r e s e n t e d i n t h i s p a p e r on t h e n e g a t i v e i o n shape and n u c l e a r - e x c i t e d F e s h b a c h r e s o n a n c e s o f t h r e e b a s i c groups o f p o l y a t o m i c o r g a n i c m o l e c u l e s , namely, n o n a r o m a t i c d o u b l e - b o n d e d s y s t e m s , b e n z e n e and b e n z e n e d e r i v a t i v e s , and s e l e c t e d h a l o c a r b o n s . The new k n o w l e d g e on N I S s p r o v i d e s f o r e f f e c t i v e p r o b i n g o f m o l e c u l a r s t r u c t u r e and r e a c t i o n s , and f o r the t e s t i n g o f t h e o r e t i c a l t r e a t m e n t s o f m o l e c u l a r e l e c t r o n i c structure; i t i s a l s o d i r e c t l y r e l e v a n t to a m u l t i p l i c i t y o f a p p l i e d f i e l d s and t e c h n o l o g i e s (_3, 4 ) . Nonaromatic Double-Bonded S t r u c t u r e s E f f e c t o f D o u b l e Bonds and T h e i r M u t u a l I n t e r a c t i o n on t h e E n e r g y o f N I S s Formed v i a a Shape R e s o n a n c e M e c h a n i s m . The l o w e s t NIS o f C H ^ i s l o c a t e d ( a d i a b a t i c ; 0 + 0 t r a n s i t i o n ) a t 1.55 eV above t h e g r o u n d s t a t e and i s due to e l e c t r o n c a p t u r e i n t o t h e 2

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.

CHRISTOPHOROU

13

Negative Ion States

(b ) orbital. S i n c e to our knowledge t h i s i s the lowest NIS o f t h e i s o l a t e d m o l e c u l e , t h e EA o f C ^ H ^ i s - 1 . 5 5 e V . C o m p a r i n g t h e p o s i t i o n s o f t h e r e p o r t e d l o w e s t RLSs OF C H ^ and C H ( T a b l e 1) i t i s seen t h a t the e t h y l e n i c d o u b l e bond s t a b i l i z e s t h e NIS (makes EA l e s s n e g a t i v e ) . The p r e s e n c e o f an a d d i t i o n a l d o u b l e bond i n t h e m o l e c u l e ( T a b l e 1) l o w e r s t h e NIS e n e r g y even m o r e . T h i s o b s e r v a t i o n a p p e a r s t o be v a l i d , a l s o , f o r c y c l i c compounds where a g a i n two d o u b l e bonds l o w e r t h e n e g a t i v e i o n r e s o n a n c e e n e r g y much more t h a n does a s i n g l e d o u b l e b o n d . Furthermore, t h i s l o w e r i n g i s a f u n c t i o n o f the m u t u a l i n t e r a c t i o n o f t h e two d o u b l e b o n d s , w h i c h , i n t u r n , depends on t h e i r r e l a t i v e p o s i tions. L o c a l i z e d d o u b l e bonds c a n i n t e r a c t d i r e c t l y t h r o u g h s p a c e o r i n d i r e c t l y t h r o u g h o t h e r bonds (2)The changes i n t h e e n e r g i e s o f the NISs a p p a r e n t l y are s e n s i t i v e probes o f such interactions. The p o s i t i o n o f th NIS i Tabl 1 fro electro scattering studies. O l i s t e d molecules is negativ knowledge o f the c r o s s s e c t i o n f o r f o r m a t i o n o f these NISs i s available. g

2

2

6

E f f e c t o f CH^ G r o u p . S u b s t i t u t i o n o f CH~ f o r H l o w e r s t h e m o l e c u l a r s y m m e t r y , and t h e d a t a i n T a b l e 2 snow t h a t i t r a i s e s t h e p o s i t i o n o f t h e NIS (makes the EA s m a l l e r ) . The same e f f e c t s a r e o b s e r v e d when one o f t h e C H ^ g r o u p s o f t h e e t h y l e n e m o l e c u l e i s r e p l a c e d by a t o m i c o x y g e n , w h i c h i s t h e u n i t e d atom e q u i v a l e n t o f C H . S i m i l a r e f f e c t s are observed for aromatics ( e . g . , the p o s i t i o n o f t h e l o w e s t NIS o f a n i l i n e , m e t h y l a n i l i n e and d i m e t h y l a n i l i n e , were r e p o r t e d t o be r e s p e c t i v e l y , - 1 . 1 3 ( 1 0 ) , - 1 . 1 9 ( 1 1 ) and - 1 . 2 4 ( J j O e V ) . These d a t a were a g a i n from e l e c t r p j s c a t t e r i n g e x p e r i m e n t s ; t h e N I S s a r e v e r y s h o r t - l i v e d (•• b

-1.3 **

Acetaldehyde

Acetone

:;>• See

f o o t n o t e ** i n T a b l e 1.

**

Maximum o f shape resonance,

a

Ref. 6.

b

Ref. 8.

c

Ref. 9.

T a b l e 3.

E f f e c t s o f e l e c t r o n w i t h d r a w i n g groups o r atoms on the p o s i t i o n o f the f i r s t NIS o f some e t h y l e n i c - t y p e s t r u c t u r e s .

Molecule

Reported p o s i t i o n o f f i r s t NIS (eV)

Formula

Ethylene

H C=CH

(-1.55)

Formaldehyde

H C=0

b (-0.65T

2

2

Oxygen

(+0.44)

C

(+2.88)

c

Comments

Isoelectronic sequence; replace CH by u n i t e d atom e q u i v a lent, 0

a

2

°2 TetracyanoethyIene

(CN) C=C(CN)

TetrachloroethyIene

ci c=cci

2

2

2

2

(+2.12?)

Nuclear-exc i t e d Feshbach C

resonance

a

See T a b l e 1

b

See T a b l e 2

c

Equated to the EA v a l u e r e p o r t e d and thus t o the p o s i t i o n o f the lowest NIS [ 0 ( R e f . 13.), C ( C N ) (Ref. 14), C C 1 (Ref. 15.)] 2

2

4

2

4

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

16

PHOTON, ELECTRON, AND ION PROBES

a

RADIAL

DISTANCE OF INCIDENT ELECTRON, p

INTERNUCLEAR

DISTANCE

Figure 1. Schematic illustration of (a) shape and (b) nuclear-excited Feshbach resonances. The symbols |0> and \R> designate, respectively, the electronic ground state of the neutral molecule and the NIS (1).

ELECTRON ENERGY (eV)

Figure 2. Negative ion autodetachment lifetime (*) and negative ion current (measured without the retarding potential difference method) (O) for C (CN)i~* as a function of electron energy (11) 2

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

CHRISTOPHOROU

Negative Ion States

°

S F

6

TETRACHLOROETHYLENE CI

\

/

/

c =c \

CI

0

0.5

CI CI

1.0

ELECTRON ENERGY, e (eV)

Journal of Chemical Physics

Figure 3. Comparison of the energy dependence of the C Cl/~* ion with that of SF ~*. The yields of the two ions were normalized at the peak (16). 2

6

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

18

PHOTON, ELECTRON, AND ION PROBES

instrumental. A t ^ 0 . 0 eV t h e l i f e t i m e o f C C 1 , toward a u t o detachment i s (_16) 14+3 u s e c . V e r y i n t e r e s t i n g l y the t r a n s i e n t C^Cl^ i o n c a n m u l t i p l y fragment^ y i e l d i n g l a r g e q u a n t i t i e s o f CI and a l s o s m a l l amounts o f C l ^ . The wqrk o f J o h n s o n e t a l . ( 1 6 ) has shown t h a t t h e t r a n s i e n t a n i o n s , M , o f c h l o r o e t h y I e n e s and c h l o r o e t h a n e s fragment ( e v e n when t h e s e a r e formed by the capture of e l e c t r o n s w i t h v i r t u a l l y zero_energy) through a m u l t i p l i c i t y of channels. D e p e n d i n g on M , a l l o r p a r t o f t h e d e c o m p o s i t i o n c h a n n e l s shown b e l o w were found (JJ>) t o be p o s s i b l e . 2

(*)

C)

(M-C1)+C1_ C1+(M-C1) (M-2C1)+C1 _ M ( l o n g - l i v e d ; >10 2

6

sec)

The c r o s s s e c t i o n f o e x i s t e n c e o f f i v e maxima b e l o w ^ 2 . 0 eV f o r the c h l o r o e t h y I e n e s (and a l s o f o r the c h l o r o e t h a n e s ) (16>). I t seems t h a t t h e s e a r e n o t due t o v i b r a t i o n a l e x c i t a t i o n , b u t r a t h e r due t o s e p a r a t e N I S s a s s o c i a t e d (_16) w i t h o r b i t a l s d o m i n a t e d by the p - o r b i t a l s o f the CI a t o m . T h e i r p o s i t i o n s a r e r e l a t i v e l y i n s e n s i t i v e , and t h e i r c r o s s s e c t i o n s v e r y s e n s i t i v e t o the d e t a i l s o f the m o l e c u l a r structure. A s i m i l a r b e h a v i o r i s e x h i b i t e d by a number o f p e r f l u o r o c a r b o n s t r u c t u r e s ( s e e R e f s . 18 and 19 and t h e f o l l o w i n g sections). A l t h o u g h f o r the m o l e c u l e s i n F i g . 4 _ g i s s o c i a t i v e a t t a c h m e n t p r o c e s s e s a r e e x p e c t e d t o be f a s t ( and V, i s a f u n c t i o n o f phonon operators. ^* T r a n s p o r t i s s t u d i e d i n terms o f the m e a n - s q u a r e d i s p l a c e ment o f an e x c i t a t i o n c r e a t e d a t the o r i g i n a t t i m e z e r o . A t l o n g t i m e s , t h e r a t e o f change o f t h i s q u a n t i t y g i v e s t h e d i f f u s i o n c o e f f i c i e n t ; we s h a l l n o t c o n s i d e r t h e t r a n s p o r t b e f o r e i t b e comes d i f f u s i v e . The m e a n - s q u a r e d i s p l a c e m e n t i s o b t a i n e d from the e x c i t a t i o n d e n s i t y m a t r i x , which i s the f u l l d e n s i t y m a t r i x , o f the c o u p l e d s y s t e m i n t e g r a t e d o v e r a l l phonon s t a t e s . Exact f o r m a l e x p r e s s i o n s f o r t h i s q u a n t i t y c a n be o b t a i n e d , b u t more t r a c t a b l e e x p r e s s i o n s f o l l o w i f o n l y terms up t o s e c o n d o r d e r i n the V ^ are r e t a i n e d . After further neglect of small q u a n t i t i e s , the d i f f u s i o n

c o e f f i c i e n t c a n be e x p r e s s e d

as

(12).

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4.

SILBEY AND MUNN

41

Exciton States and Transport in Crystals

where t h e d o u b l e a n g l e b r a c k e t s d e n o t e a t h e r m a l a v e r a g e o v e r e x c i t a t i o n s t a t e s o f energy and ^ i s t h e v e l o c i t y The the r a t e s o f s c a t t e r i n g out o f s t a t e so t h a t t h e f i r s t t e r m i n E q . ( 2 . 3 ) h a s t h e u s u a l form f o r band t r a n s p o r t , and t h e form o f h o p p i n g r a t e s . Both and Y ^ ^ o b t a i n e d from q u a n t i t i e s a

n

r

e

a

v

W

e

t

n

e

kk';qs=

< V

sk-

( T )

a

r

J ^i


o

e

k

e

i

(

E

q

s

"

E

k

,

)

T

+

i ( E k _ E q ) T }

V° "

( 2

'

4 )

The

s i n g l e angle Jyracket (T)=e ph V^ e ph " i v e n by ^ l H

T

(

' k k * " " ^q q q s k k and t h e h o p p i n g r a t e s a r e g i v e n by 2 2 q,q+K;k,k+K|

2

-

5

)

K = ( )

( o r v a r i o u s e q u i v a l e n t forms (6^, 12). D e r i v a t i o n o f t h e d i f f u s i o n c o e f f i c i e n t thus r e q u i r e s e v a l u a t i o n o f the c o r r e l a t i o n functions < V , , V ( T ) > , o f t h e i n t e g r a l s i n E q . ( 2 . 4 ) , o f t h e sums i n E q s . ( 2 ? 5 ) an*d ( 2 . 6 ) , and o f t h e t h e r m a l a v e r a g e i n E q . ( 2 . 3 ) . k

Untransformed

Coupling

General R e s u l t s . I f the electron-phonon c o u p l i n g i s s u f f i c i e n t l y weak, t h e l a s t t e r m i n E q . ( 2 . 1 ) c a n be t r e a t e d as a perturbation. Then i n E q . ( 2 . 2 ) t h e o p e r a t o r V, i s g i v e n by icq V

=

kq 8 . k

The

q

. (b _

k

q

k

q

+

b:

k

+

q

).

(3.1)

required c o r r e l a t i o n functions

< V

V

k K,q K qk +

( T ) >

=

+

L(n _^+l)e k

where we have u s e d

^

a r e o f t h e form

X

K-ql^-q ^ - q

6

the r e s u l t s

q

(3.2)

g =g and to =u) . -q q -q q

In Eq. (3.2)

6

American Chemical Society Library 1155 16th St. N. w. Washington, D. C. 20036

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

48

PHOTON, ELECTRON, AND ION PROBES

n^_ i s the g i v i n by V , where

=

gq g>

"

thermal

n

(

3= l/kg

)

V q T

t

n

e

e q u i l i b r i u m number



6 w k

(e

o f phonons

i n mode k - q ,

- * q ^ ^ ^ s c a t t e r i n g r a t e s are then (j

=(J

a n c

n

=

2

2

n

o r

a

T

r ° £ = 27Tg o) [(n+l)N

(E, -to)+nN ex

K.K

e

(E, +u>)], ex

K.

(3.4)

K.

where N ( E ) i s the e x c i t a t i o n d e n s i t y o f s t a t e s . I n g e n e r a l the r e s u l t i n g d i f f u s i o n c o e f f i c i e n t i s d i f f i c u l t t o o b t a i n e x c e p t by n u m e r i c a l means. Howeve s u c h t h a t B>>to, we c a n f

kk

^27rg

2

2 a )

(2n+l)N (E ) e x

(3.5)

k

where m* i s t h e e f f e c t i v e m a s s . l e a d to D/a

2

5

2

Conventional procedures

2

= [27Tk T/(m*) ]^/g o) (2n+l)

(L3)

then

(3.7)

B

where^a^is the i n t e r m o l e c u l a r d i s t a n c e . ^ . ^ S i n c e m * ^ l / B , then D^B /to . The m o b i l i t y y=e$D v a r i e s as T ' at low t e m p e r a t u r e s such t h a t k T«u>, changing e v e n t u a l l y to T when k _ T » w . /t

A c o u s t i c Phonons. We assume t h a t a c o u s t i c phonons c a n be a d e q u a t e l y d e s c r i b e d by a Debye s p e c t r u m c u t - o f f f r e q u e n c y and the e l e c t r o n - p h o n o n c o u p l i n g i s g i v e n by t h e deformation p o t e n t i a l approximation g

q

=

The s c a t t e r i n g

(3.8) rate

is

then

r £ = 2 7 r A N ~ y > { ( n +1)6(E. -E,+w ) + kk q q k-q k q a

n

q

1

6(E, -E,-co )} k-q k q

(3.9)

I n some l i m i t s , r e s u l t s c a n be o b t a i n e d more d i r e c t l y . In the conventional semiconductor limit B>>k T»w^ we have n % k T / ( A ) >>1, and | E _ - E ^ | »/k

T(m*) ]V

(3.11)

5

15

5/2

49

3/2

so t h a t D'vB /A^B i f we assiime A i s p r o p o r t i o n a l t o B. Eq. (3.11) g i v e s the standard T temperature dependence f o r the mobility (13). In molecular crystals one may a l s o require the limit k T>>u) >>B. Then i n E q . ( 3 . 9 ) n o n z e r o c o n t r i b u t i o n s a r i s e o n l y f o r f r e q u e n c i e s u) ^ ( ^ B a ) , we o b t a i n Z

k

N p

(o))=3w t

l

/u) .

Taking

D/a =a)^/127rAk T, 2

(3.13)

B

3

similarly

D

3

so t h a t D ^ a ^ / A ^ o ^ / B ^ w i t h t h e p r e v i o u s a s s u m p t i o n A B) and t h e m o b i l i t y v a r i e s as T . The n a r r o w e x c i t a t i o n b a n d i n t h i s l i m i t g r e a t l y r e d u c e s t h e number o f a l l o w e d o n e - p h o n o n s c a t t e r i n g p r o c e s s e s , so t h a t t h e d i f f u s i o n c o e f f i c i e n t may b e l a r g e , a s t h e r a t i o ^ /B i n d i c a t e s . Finally, i n t h e l i m i t k T>>B>>u) , t h e s c a t t e r i n g r a t e s a r e g i v e n by E q . ( 3 . 1 0 ) b u t t h e t h e r m a l a v e r a g e s i n D hay^e t o b e ^ t a k e n over a narrow excitation band. We take v, ^(%Ba) and N ( E M / B , obtaining 0C

D

B

e

x

D

k

D/a =B /(167rAk 2

3

15

T)

(3.14) -2

so t h a t t h e m o b i l i t y v a r i e s as T . T h i s r e s u l t accords with the n a r r o w e x c i t a t i o n band t r e a t m e n t s o f G l a r u m ( 1 4 ) and F r i e d m a n (15). _

Transformed Coupling Transformation. The t r a n s f o r m a t i o n o f t h e H a m i l t o n i a n ( 2 . 1 ) w h i c h y i e l d s a weak r e s i d u a l e x c i t a t i o n - p h o n o n c o u p l i n g e v e n when t h e g a r e l a r g e h a s b e e n d i s c u s s e d s e v e r a l t i m e s ( 4 , T_ 16^> 1 7 ) . I t p r 8 d u c e s a u n i f o r m s h i f t i n t h e e x c i t a t i o n e n e r g y l e v e l s and a d i s p l a c e m e n t i n t h e e q u i l i b r i u m p o s i t i o n o f t h e phonons c o r r e s p o n d i n g to t h e f o r m a t i o n o f a p o l a r o n . Since the t r a n s f e r interactions J compete w i t h t h i s t e n d e n c y t o f o r m a l o c a l i z e d s t a t e , t h e optimum t r a n s f o r m a t i o n s h o u l d be d e t e r m i n e d v a r i a t i o n 9

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

50

PHOTON, ELECTRON, AND ION PROBES

a l l y ^6, J 7 ) . H o w e v e r , f o r p r e s e n t p u r p o s e s we use t h e full c l o t h i n g t r a n s f o r m a t i o n w h i c h i s e x a c t f o r J=0 and y i e l d s t h e c o r r e c t u n t r a n s f o r m e d r e s u l t s f o r l a r g e J and weak c o u p l i n g . The r e s u l t s a r e q u a l i t a t i v e l y s i m i l a r t o t h o s e w h i c h w o u l d be o b t a i n ed w i t h the f u l l v a r i a t i o n a l t r a n s f o r m a t i o n , b u t a r e s i m p l i f i e d by the a b s e n c e o f the t e m p e r a t u r e - d e p e n d e n t v a r i a t i o n a l p a r a meters . After the transformation, the excitation part of the Hamiltonian (2.2) is 2

H

- * ( - r t c * |g | z \e*'%n) k q n i s a l a t t i c e v e c t o r and

e x

e

where R ^n

+

aja

(4.1)

k


h n + h , n n+h n

(4.2)

j l U

J

= exp [ N

1 / 2

E(g q

Y

n

q

q


B. The s e c o n d t e r m i n E q . ( 4 . 6 ) r e p r e s e n t s t h e h o p p i n g c o n t r i b u t i o n , which i n c r e a s e s w i t h i n c r e a s i n g temperature through I g ( y ) and t h e e x p o n e n t i a l f a c t o r . H o w e v e r , once B becomes much l e s s t h a n f,^ t h e e x p o n e n t i a l f a c t o r becomes c o n s t a n t , and t h e d e c r e a s e i n B o u t w e i g h s t h e i n c r e a s e i n I ( y ) , so t h a t e v e n t u a l l y the second term a l s o d e c r e a s e s w i t h i n c r e a s i n g t e m p e r a t u r e . The o c c u r r e n c e o f t h i s d e c r e a s e i s d i s c u s s e d i n more d e t a i l b e l o w . I n the l i m i t f>>B assumed i n most p r e v i o u s w o r k , we f i n d , D/a

2 z

=

f j—i (2^) [l (y)-l]

^[I (y)-1]B 2 4f

2

0

+

J s

0

(

4

?

J

T h i s i s e s s e n t i a l l y o f t h e form d e r i v e d e a r l i e r ( 4 ) . We n o t e t h a t i n E q . ( 4 . 7 ) f, o r e q u i v a l e n t l y A, c a n n o t t e n d t o z e r o ( w h i c h w o u l d make D i n f i n i t e ) , b e c a u s e t h e e q u a t i o n i s v a l i d o n l y f o r f>>B. On t h e o t h e r h a n d , J( c a n t e n d t o z e r g , i n w h i c h c a s e o n l y t h e band t e r m r e m a i n s . A s t h e v e l o c i t i e s v i n E q . ( 2 . 3 ) tend to z e r o , so do t h e s c a t t e r i n g r a t e s £ • b o t h f a c t o r s v a r y as % , so t h a t t h e i r r a t i o t e n d s to a c o n s t a n t . Le >e written

as 2

D/a

2

= (

B

Q 8Ag 2 2

(4.8)

2

sinh | ^ exp(-2g tanh

2

I n the

opposite

D/a

2

limit

B

- L _ ^[l (y)-l] x

Q

> >

T

>

we o b t a i n t h e

+ 4

new

[T(y)-1] °

result

Be-

B 2

«

2 / 8

H e r e f does n o t o c c u r and so c a n t e n d t o z e r o , b e i n g a l r e a d y assumed much s m a l l e r t h a n B . I f t h e n B t e n d s to z e r o , the d i f f u s i o n c o e f f i c i e n t v a n i s h e s , as e x p e c t e d . V i b r a t i o n a l d i s p e r s i o n ( n o n z e r o A ) i s t h e r e f o r e not e s s e n t i a l to o b t a i n d i f f u s i v e m o t i o n and f i n i t e t r a n s p o r t c o e f f i c i e n t s i n t h i s l i m i t , b u t to show t h i s has r e q u i r e d a s u f f i c i e n t l y g e n e r a l t h e o r y . Discussion I n t h i s p a p e r we have shown how e l e c t r o n i c t r a n s p o r t in molecular crystals can be t r e a t e d much more g e n e r a l l y than hitherto. The p r e s e n t t r e a t m e n t a v o i d s ( i ) t h e a s s u m p t i o n t h a t v i b r a t i o n a l r e l a x a t i o n i s f a s t compared w i t h e x c i t a t i o n t r a n s f e r

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTON, ELECTRON, AND ION PROBES

52

(f>>B, as u s u a l l y assumed i n p o l a r o n and e x c i t o n transport t h e o r i e s ) , and ( i i ) the a s s u m p t i o n t h a t t h e e x c i t a t i o n - p h o n o n c o u p l i n g i s weak (gtjon

A

V

(2)

and a mean h o p p i n g i n t e g r a l , V . V a r i a t i o n s i n t h e s i t e e n e r g i e s from the mean ( d e s c r i b e d by A ) a r e r e f e r r e d t o as " d i a g o n a l d i s o r d e r " w h e r e a s a n a l o g o u s v a r i a t i o n s o f the h o p p i n g i n t e g r a l s are c a l l e d " o f f - d i a g o n a l d i s o r d e r . " S i m i l a r i l y , i f these v a r i a t i o n s a r e c a u s e d by l o c a l t i m e - i n d e p e n d e n t f l u c t u a t i o n s i n comp o s i t i o n or s t r u c t u r e one s p e a k s o f " s t a t i c d i s o r d e r " w h i l e i f

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

68

PHOTON, ELECTRON, AND ION PROBES

t h e y a r e g e n e r a t e d by t h e r m a l v i b r a t i o n s one e m p l o y s the t e r m "dynamic d i s o r d e r . " Whereas s t a t i c d i a g o n a l and o f f - d i a g o n a l d i s o r d e r c a n o c c u r i n p o l y m e r s , o n l y dynamic d i s o r d e r s h o u l d be present i n molecular c r y s t a l s . A m o d e l p r o p e r l y embodying dynami c d i s o r d e r i s more c o m p l i c a t e d t h a n E q . ( 1 ) ( 3 , 8, 9). The i m p o r t a n t p o i n t f o r our p r e s e n t p u r p o s e , h o w e v e r , i s t h e r e c o g n i t i o n t h a t a l l s o u r c e s o f d i s o r d e r must be c o n s i d e r e d i n o r d e r t o d e t e r m i n e t h e l o c a l i z e d or e x t e n d e d c h a r a c t e r o f t h e e l e c t r o n i c s t a t e s a s s o c i a t e d w i t h charges (or e x c i t o n s ) i n j e c t e d i n t o p o l y mers and m o l e c u l a r m a t e r i a l s , and t h a t t h e s e s o u r c e s o f d i s o r d e r a r e d e s c r i b e d by c h o o s i n g s u i t a b l e d i s t r i b u t i o n s o f t h e and V , p a r a m e t e r s i n E q . ( 1 ) o r an a p p r o p r i a t e generalization tRereof. I n terms o f the s i m p l i f i c a t i o n o f E q . ( 1 ) d e f i n e d b y c o n s i d e r i n g only nearest-neighbor hopping i n t e g r a l s ( i . e . , n e g l e c t ing long-range hops), i t c a n be shown ( 2 5 26) t h a t injected charges are localized a n i o n s w i t h i n the s o l i d A>czV

(3)

i n w h i c h z i s the c o o r d i n a t i o n number o f the ( p r e s u m a b l y i d e n t i c a l ) m o l e c u l a r s i t e s and c i s a d i m e n s i o n l e s s number o f the o r d e r o f u n i t y w h i c h depends b o t h on t h e c o n n e c t i v i t y ( i . e . , d i m e n s i o n a l i t y ) o f t h e m o l e c u l a r s y s t e m and on the e x t e n t o f o f f - d i a g o n a l disorder. T y p i c a l l y c - 2.5 for ( i s o t r o p i c ) three dimensional systems, 1.5 f o r two d i m e n s i o n a l s y s t e m s , and z e r o f o r onedimensional systems. I n e q u a l i t y 03) i s b e l i e v e d t o be s a t i s f i e d i n most p o l y m e r s : I n t h i s case charges i n j e c t e d i n t o the polymer form l o c a l , m o l e c u l a r a n i o n s and c a t i o n s , r e s p e c t i v e l y , r a t h e r than extended m o b i l e s t a t e s l i k e those c h a r a c t e r i s t i c o f c r y s t a l l i n e covalent semiconductors. C o n s e q u e n t l y , under these c i r c u m s t a n c e s t h e e l e c t r o n s and h o l e s a r e more a c c u r a t e l y v i s u a l i z e d as i o n s i n s o l u t i o n w h i c h move, h o w e v e r , by c a r r i e r h o p p i n g r a t h e r t h a n i o n i c d i f f u s i o n . On t h e o t h e r h a n d , s t a t i c d i a g o n a l d i s o r d e r s h o u l d be z e r o i n b u l k o r g a n i c c r y s t a l s , so t h a t i n e q u a l i t y ( 3 ) i s n o t n e c e s s a r i l y s a t i s f i e d and e x t e n d e d b a n d - l i k e s t a t e s a r e n o t ruled out. I n any c a s e , i f A and V a r e known, an a s s e s s m e n t o f t h e l o c a l i z a t i o n o f t h e i n j e c t e d c h a r g e s c a n be made. D i f f e r e n t c l a s s e s o f p o l y m e r s and m o l e c u l a r j s o l i d s e x h i b i t s y s t e m a t i c a l l y d i s t i n c t r a n g e s o f v a l u e s o f A and V . F o r t y p i c a l p e n d a n t - g r o u p and m o l e c u l a r l y doped p o l y m e r s , O . l e V A leV, V 8 ) . A s i m i l a r s i t u a t i o n seems t o p r e v a i l f o r m o l e c u l a r glasses, a l t h o u g h the p a r a m e t e r v a l u e s a r e n o t y e t f i r m l y e s t a b l i s h e d i n this case. For b u l k m o l e c u l a r c r y s t a l s i n the absence o f d e f e c t s

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6.

DUKE

Localization of Electronic States

69

t h e s t a t i c d i s o r d e r c o n t r i b u t i o n t o A v a n i s h e s so one e x p e c t s A = C'KT i n w h i c h c is a constant. T h i s o b s e r v a t i o n l e d to the prediction (_1) and s u b s e q u e n t o b s e r v a t i o n (_3, 2 7 , 28) o f a t r a n s i t i o n w i t h d e c r e a s i n g t e m p e r a t u r e from h o p p i n g m o t i o n b e tween l o c a l i z e d s t a t e s to band m o t i o n i n e x t e n d e d s t a t e s i n n a p h t h a l e n e and d e u t e r a t e d n a p h t h a l e n e . F i n a l l y , for segregatedstack l i n e a r c h a i n charge t r a n s f e r s a l t s ( e . g . , TTF-TCNQ) and non-saturated-backbone p o l y m e r s ( e . g . , p o l y ( s u l f u r n i t r i d e ) and p o l y a c e t y l e n e ) A < V so t h a t t r a n s p o r t v i a d e l o c a l i z e d s t a t e s i s p o s s i b l e p r o v i d e d the e l e c t r o n i c m o t i o n i s not s t r i c t l y one dimensional. I n some c a s e s , h o w e v e r , the q u a s i - o n e - d i m e n s i o n a l n a t u r e o f the m a t e r i a l s s t r o n g l y s u g g e s t s the o c c u r r e n c e o f Fermi-surface i n s t a b i l i t i e s (10-15, 29). For example, i n p o l y a c e t y l e n e P e i e r l ' s i n s t a b i l i t i e s , which manifest themselves as b o n d - a l t e r n a t i n g backbone s t r u c t u r e s i n undoped m a t e r i a l s , seem t o p r e d o m i n a t e ( L 5 , _16) • I t f u r t h e r has b e e n s u g g e s t e d (_L3 1 4 30) t h a t these i n s t a b i l i t i e upon d o p i n g t h a n t h o s e w h i c semiconduc t o r s . We c o n c l u d e , t h e r e f o r e , t h a t a l t h o u g h t h e o p t i c a l and t r a n s port properties o f o r g a n i c p o l y m e r s and m o l e c u l a r s o l i d s may a p p e a r a n a l o g o u s to t h o s e o f t y p i c a l c o v a l e n t s e m i c o n d u c t o r s , t h e m a t e r i a l s s h o u l d n o t be t h o u g h t o f as " o r g a n i c s e m i c o n d u c t o r s " i n the J i r a d i t i o n a l o n e - e l e c t r o n s e n s e . Most commonly ( i . e . , when A>> V) t h e y a r e more a c c u r a t e l y r e g a r d e d as F e r m i g l a s s e s c h a r a c t e r i z e d by l o c a l i z e d i n t r i n s i c a n d / o r e x t r i n s i c m o l e c u l a r i o n s t a t e s w i t h e n e r g i e s near the Fermi e n e r g y . Sometimes ( e . g . , i n t h e c a s e o f p o l y a c e t y l e n e ) t h e y a r e b e s t c o n s i d e r e d t o be o r g a n i c m e t a l s w h i c h have become s e m i c o n d u c t o r s b e c a u s e o f F e r m i - s u r f a c e instabilities. O c c a s i o n a l l y , as i n t h e c a s e o f u l t r a p u r e n a p h t h a l e n e and d e u t e r o n a p h t h a l e n e a t T £_100K, o r g a n i c c r y s t a l s do seem to e x h i b i t t r a d i t i o n a l o n e - e l e c t r o n e n e r g y - b a n d transport. T h i s s i t u a t i o n i s , however, the e x c e p t i o n r a t h e r than the r u l e . f

C o n t a c t Charge Exchange An important p r a c t i c a l problem which r e q u i r e s for i t s s o l u t i o n the concepts developed i n the p r e c e e d i n g s e c t i o n i s t h a t o f c o n t a c t c h a r g e exchange b e t w e e n i n s u l a t o r s . Long v i e w e d i n terms o f the t r a d i t i o n a l s e m i c o n d u c t o r m o d e l (31^, 3 2 ) , c h a r g e i n j e c t i o n and m o t i o n i n h i g h p o l y m e r s have s u c c e s s f u l l y e l u d e d q u a n t i t a t i v e i n t e r p r e t a t i o n u n t i l t h e r e c o g n i t i o n i n r e c e n t y e a r s (_5, 6, 18, 19) t h a t the p o l y m e r i c m a t e r i a l s i n v o l v e d a r e b e s t d e s c r i b e d as Fermi glasses. Specifically, the l o c a l i z a t i o n of injected c h a r g e s p r e d i c t e d by the F e r m i - g l a s s m o d e l i s r e q u i r e d b o t h t o u n d e r s t a n d t h e good i n s u l a t i n g p r o p e r t i e s o f p o l y m e r i c m a t e r i a l s ( 3 2 ) and to i n t e r p r e t the n o n - e q u i l i b r i u m f e a t u r e s o f c o n t a c t c h a r g e exchange C 7 , 1 8 , _19, 3 3 , 3 4 ) . The b a s i c c o n s t r u c t o f the F e r m i - g l a s s m o d e l i s a s p a t i a l l y a v e r a g e d d e n s i t y o f d o n o r and a c c e p t o r s t a t e s w h i c h i s a f u n c t i o n

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

70

PHOTON, ELECTRON, AND ION PROBES

of energy. T h i s s t a t e d e n s i t y e i t h e r may be c a l c u l a t e d from t h e m i c r o s c o p i c models n o t e d above ( 2 , 6 , _7> .§> 2) ^ ~ mined p h e n o m e n o l o g i c a l l y v i a p h o t o e m i s s i o n ( 8 ) or m e t a l - p o l y m e r c o n t a c t c h a r g e exchange (6, 8 , 1 8 , 19) m e a s u r e m e n t s . Once t h e d e n s i t i e s o f s t a t e a r e known f o r two i n s u l a t o r s , t h e i r steadys t a t e c o n t a c t c h a r g e exchange c a n be p r e d i c t e d (_7). A phenomenol o g i c a l v e r s i o n o f t h i s F e r m i - g l a s s m o d e l has b e e n shown t o p r e d i c t s u c c e s s f u l l y the c o n t a c t c h a r g e exchange o f v a r i o u s c o p o l y m e r s o f p o l y s t y r e n e and p o l y ( m e t h y l m e t h a c r y l a t e ) , and o f c a r b o n - b l a c k p i g m e n t e d p o l y m e r s (J)The m o d e l a l s o has b e e n u s e d to i n t e r p r e t q u a n t i t a t i v e l y p h o t o e m i s s i o n and UV a b s o r p t i o n d a t a for polystyrene and p o l y ( 2 - v i n y l p y r i d i n e ) (2, 8), and to c a l c u l a t e hopping a c t i v a t i o n energies i n p o l y ( 2 - v i n y l p y r i d i n e ) (35). C o n s e q u e n t l y , i t c o n s t i t u t e s the f i r s t s u c c e s s f u l a t t e m p t to p r o v i d e a s i m p l e , u n i f i e d t h e o r y o f o p t i c a l a b s o r p t i o n , c h a r g e i n j e c t i o n , and c h a r g e t r a n s p o r t i n h i g h p o l y m e r s o

r

m

a

v

e

d

e

t

e

r

Abstract The extended or localized character of charges injected into solids is determined by a competition between fluctuations in the energies of these charges from one site to another and their probability of extending over more than one site. We illustrate for trinitrofluorenone and triphenylamine the fact that injected charges can exhibit widely different transport behaviors in condensed molecular systems characterized by similar geometrical and electronic structures. A model of charge localization in condensed molecular solids is developed. This model embodies distributions of site energies and of electronic overlap integrals. A procedure for estimating these distributions using spectroscopic data is indicated and applied to a variety of polymers and molecular solids. The model predicts the systematic occurrence of localized states in certain types of materials (e.g., aromatic pendant-group polymers like polystyrene) but not in others (e.g., crystalline, non-saturated-backbone polymers like polyacetylene). Fermi-surface instabilities occur, however, in linear polymer systems which exhibit extended states. A survey is given of the varied behaviors expected for localized and extended-state polymeric materials. As an example of their utility, the results of the analysis are applied to an important practical problem: the description of contact charge exchange between polymeric insulators.

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Duke, C.B., Surface Sci., 1978, 70, 674.

2.

Duke, C.B., Mol. Cryst. Liq. Cryst., 1979, 50, 63.

3.

Duke, C.B. and Schein, L.B., Physics Today, 1980, 33(2), 42.

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6.

DUKE

71

Localization of Electronic States

4.

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5.

Duke, C.B. and Fabish, T.J., Phys. Rev. Lett., 1976, 37, 1075.

6.

Duke, C.B., Fabish, T.J. and Paton, A., Chem. Phys. Lett., 1977, 49, 133.

7.

Duke, C.B. and Fabish, T.J., J. Appl. Phys., 1978, 49, 315.

8.

Duke, C.B., Salaneck, W.R., Fabish, T.J., Ritsko, J.J., Thomas, H.R. and Paton, A., Phys. Rev. B., 1978, 18, 5717.

9.

Duke, C.B., in "Tunneling in Biological Systems", B. Chance et al., eds. (Academic Press New York 1979) pp 31-66

10. Rice, M.J., Duke, Commun., 1975, 17, 1089.

Lipari,

(Verlag Chemie,

,

11. Madhukar, A., Chem. Phys. Lett., 1974, 27, 606. 12. Rice, M.J., Solid State Commun., 1977, 21, 757; 1978, 25, 1083. 13. Rice, M.J., Phys. Lett., 1979, 71A, 152. 14. Su, W.P., Schrieffer, J.R. and Heeger, A.J., Phys. Rev. Lett., 1979, 42, 1678. 15. Kertesz, M., Koller, J. and Azman, A., J. Chem. Phys., 1977, 67, 1180; 1978, 68, 2779. 16. Duke, C.B., Paton, A., Salaneck, W.R., Thomas, H.R., Plummer, E.W., Heeger, A.J. and MacDiarmid, A.G., Chem. Phys. Lett., 1978, 59, 146. 17. Fabish, T.J., Saltsburg, H.M. and Hair, M.L., J. Appl. Phys., 1976, 47, 930; 1976, 47, 940. 18. Fabish, T.J. and Duke, C.B., J. Appl. Phys., 1977, 48, 4256; in "Polymer Surfaces", D.T. Clark and W.J. Feast, eds. (Wiley-Interscience Chichester, 1978), pp. 109-119. 19. Duke, C.B., J. Vac. Sci. Technol., 1978, 15, 157. 20. Salaneck, W.R., Phys. Rev. Lett., 1978, 40, 60. 21. Hoegl, H., J. Phys. Chem., 1965, 69, 755.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTON, ELECTRON, AND ION PROBES

72

22. Gill, W.D., J. Appl. Phys., 1972, 43, 5033; in "Photoconductivity and Related Phenomena", J. Mort and D. Pai, eds. (Elsevier, Amsterdam, 1976), pp. 303-334. 23. Pfister, G., Phys. Rev. B., 1977, 16, 3676. 24. Duke, C.B., in "Synthesis and Properties of Low-Dimensional Materials", J.E. Miller and A.J. Epstein, eds. (New York Academy of Sciences, New York, 1978), pp. 166-178. 25. Anderson, P.W., Rev. Mod. Phys., 1978, 50, 191. 26. Weaire, D. and Srivastave, V., in "Amorphous and Liquid Semiconductors", W.E. Spear, Ed. (G.G. Stevenson, Ltd., Dundee, 1977), pp. 286-290. 27. Schein, L.B., Duke 1978, 40, 197. 28. Schein, L.B. and McGhie, A.R., Phys. Rev. B., 1979, 20, 1631. 29. Chaikin, P.M., in "Synthesis and Properties of Low Dimensional Materials", J.S. Miller and A.J. Epstein, eds. (New York Academy of Sciences, New York, 1978), pp. 128-144. 30. Epstein, A.J. and Miller, J.S., Solid State Commun., 1978, 27, 325. 31. Harper, W.R., "Contact and Frictional Electrification", (Clarendon Press, Oxford, 1967), pp. 185-222; 245-268. 32. Wintle, H.F., Trans. IEEE., 1977, EI-12, 97. 33. Ruckdeschel, F.R. and Hunter, L.P., J. Appl. Phys., 1975, 46, 4416. 34. Lowell, J., J. Phys. D: Appl. Phys., 1976, 9, 1571; 1979, 12, 1541. 35. Duke, C.B. and Meyer, R.J., Bull Am. Phys. Soc., 1979, 24, 309; Phys. Rev., (submitted). RECEIVED December 22,

1980.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

7 Electronic Structure of Some Simple Polymers and of Highly Conducting and Biopolymers J. LADIK, S. SUHAI, and M . SEEL University Erlangen-Nürnberg, D-8520 Erlangen, West Germany, and Department of Applied Mathematics, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1

Method The ab i n i t i o SCF LCAO a p p l i e s a n o n - l o c a l exchange and f o u r c e n t e r i n t e g r a l s i f i n t o a c c o u n t has b e e n c h o s e n ) (1_). I n t h i s t h e o r y one h a s equation

c r y s t a l o r b i t a l (CO) method ( w h i c h and k e e p s a l l t h e o c c u r r i n g t h r e e t h e number o f n e i g h b o u r s t o be t a k e n was d e v e l o p e d a b o u t t w e l v e y e a r s ago to s o l v e the g e n e r a l i z e d e i g e n v a l u e

!(£)d.Oc) = . ( S ) § ( £ ) d . ( £ )

(l)

e

where

_^ ikR+

ikR-* |(k)

(

(

i ?

q

=£e

)

)

(I(q»

gtf)

,

->•

->•

q

q

r , s = < X

r

>

s

5 r

| x

?

§(q)

(2)

3a

>

s

< >

X V |x N

- < X °|F|x ^ > r

q

§(£) = £ e

5

r

s

>

s

$' U

P(^4 )

x «

2

O( )

X r

1

q i X u

u,v \ ( 2 )

q

i

| ^ l x 12

v

( D

2

(2)|^-| /(l) 12 x

q 2 X v

q

x (2)>)

V

1

2

_

(2)>-^< /(l) x

(3b)

s

Here y ^ i s t h e s - t h AO i n the u n i t c e l l c h a r a c t e r i z e d by the s p o s i t i o n v e c t o r R> and t h e g e n e r a l i z e d c h a r g e - b o n d o r d e r m a t r i x e l e m e n t i s g i v e n t^r the e x p r e s s i o n (1): n* (4) c

P

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l

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=

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J

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d

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d

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)

e

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0097-6156/81/0162-0073$05.00/0 © 1981 American Chemical Society In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

^

74

PHOTON, ELECTRON, AND ION PROBES

where w s t a n d s f o r the v o l u m e o f the f i r s t B r i l l o u i n zone i n r e c i p r o c a l s p a c e , %. i s the c r y s t a l momentum and n * d e n o t e s t h e number o f f i l l e d b a n d s . I t was e a s y to show t h a t we c a n f o r m u l a t e t h e method a l s o i n t h e c a s e o f a combined symmetry o p e r a t i o n ( f o r i n s t a n c e h e l i x operation = t r a n s l a t i o n + rotation) instead of simple t r a n s l a t i o n (2^). I n t h i s c a s e k i s d e f i n e d on the combined symmetry o p e r a t i o n and from g o i n g f r o m one c e l l t o the n e x t one, one has ( 1 ) t o p u t t h e n u c l e i i n t h e p o s i t i o n s r e q u i r e d by the symmetry o p e r a t i o n and ( 2 ) one has t o r o t a t e a c c o r d i n g l y a l s o the b a s i s s e t . F i n a l l y i t s h o u l d be m e n t i o n e d t h a t e f f i c i e n t techniques have been d e v e l o p e d t o h a n d l e l o n g r a n g e e l e c t r o s t a t i c e f f e c t s i n polymers using a modified m u l t i p o l e expansion ( 3 ) . P o l y a c e t y l e n e s and P o l y d i a c e t y l e n e s The d e s c r i b e d metho ( =CH^

C H = ) and

trans-cisoid

CH=CH (—CH

CH—) p o l y a c e t y l e n e

and

to

polydiacetylene

(ideal

CH—CH acetylene structure). F o r the p o l y a c e t y l e n e s t h e g e o m e t r y e s t a b l i s h e d by I t o h et a l ( 4 ) and f o r p o l y d i a c e t y l e n e the g e o m e t r y p u b l i s h e d by Chance e_t a_l (_5) has been u s e d . F o r t h e c a l c u l a t i o n s an ST0-3G b a s i s s e t (6) has b e e n a p p l i e d ( u s i n g an ST0-4G b a s i s s e t no s i g n i f i c a n t change has been found i n t h e band s t r u c t u r e s ) . F o r the p o l y a c e t y l e n e s i x t h n e i g h b o u r ' s i n t e r a c t i o n s and f o r t h e p o l y d i a c e t y l e n e f i f t h n e i g h b o u r s i n t e r a c t i o n s have been t a k e n into account. I n T a b l e 1 we show the c o n d u c t i o n and v a l e n c e b a n d s of these systems. As we c a n see from t h e T a b l e a l l t h e t h r e e c h a i n s ( a n d t h i s i s the c a s e a l s o f o r f u r t h e r two o t h e r p o l y a c e t y l e n e and t h r e e p o l y d i a c e t y l e n e c h a i n s ( 7 ) w h i c h a l s o have been c a l c u l a t e d ) have b r o a d v a l e n c e and c o n d u c t i o n bands w i t h w i d t h s b e t w e e n 4 . 4 and 6 . 5 eV-s. C o m p a r i n g the band s t r u c t u r e o f the two p o l y a c e t y l e n e c h a i n s we c a n f i n d t h a t t h e p o s i t i o n o f the bands and t h e i r w i d t h s i s n o t v e r y s t r o n g l y i n f l u e n c e d by the d i f f e r e n t g e o m e t r i e s . This i s a g a i n the c a s e i f we compare the h e r e n o t d e s c r i b e d band structures of the further p o l y a c e t y l e n e and p o l y d i a c e t y l e n e chains. On the o t h e r hand the p o s i t i o n o f the v a l e n c e and c o n d u c t i o n bands and the w i d t h s o f the v a l e n c e bands o f the p o l y d i a c e t y l e n e c h a i n s i s more d i f f e r e n t from t h o s e o f the p o l y acetylene chains. To c o n c l u d e we c a n say t h a t due t o the b r o a d v a l e n c e and c o n d u c t i o n bands o f t h e s e s y s t e m s ( w h i c h mean r a t h e r l a r g e h o l e and e l e c t r o n m o b i l i t i e s , r e s p e c t i v e l y ) one c a n e x p e c t t h a t i f doped w i t h e l e c t r o n a c c e p t o r s o r d o n o r s t h e s e s y s t e m s w i l l become good c o n d u c t o r s , w h i c h i s , as i t i s e x p e r i m e n t a l l y e s t a b -

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

7.

LADIK ET AL.

75

Electronic Structure of Polymers Table

I

The C o n d u c t i o n and V a l e n c e Band o f c i s - t r a n s o i d and t r a n s - c i s o i d P o l y a c e t y l e n e s and o f P o l y d i a c e t y l e n e ( I d e a l A c e t y l e n e S t r u c ture) i n eV's V a l e n c e Band e . mm

z

c-t

-9.75

-4.52

t-c

-9.64

-4.56

C o n d u c t i o n Band (

6e

min

max

5,.23

3.30

9.77

6.47

5,.08

3.65

9.71

6.06

max

Polyacetylene

Polydiacetylene -8.08

t

Bandwidth

l i s h e d , the case. I n t h i s r e s p e c t one s h o u l d m e n t i o n t h a t a (CH ) or a ( S i H ) c h a i n h a s a l s o b r o a d v a l e n c e and c o n d u c t i o n b a n d s * (£$). S i n c e £ h e s e c h a i n s do n o t c o n t a i n 7t e l e c t r o n s , one w o u l d need more p o w e r f u l a c c e p t o r s ( l i k e a l k a l i m e t a l i o n s ) t o t a k e o u t e l e c t r o n s from t h e v a l e n c e b a n d . I n t h e a u t h o r s ' o p i n i o n t h i s c o u l d be done and i n t h i s way t h e s e c h a i n s c o u l d become a l s o good c o n d u c t o r s ( s i n c e a c c e p t o r s n e v e r t a k e o u t a f u l l e l e c t r o n i c charge per bond, but o n l y a s m a l l f r a c t i o n o f i t , the chances t h a t i n t h i s way t h e a bonds o f t h e c h a i n w i l l b e b r o k e n i s v e r y small). T u r n i n g now t o l a r g e r systems t h e ab i n i t o SCF LCAO CO method h a s been a p p l i e d a l s o t o n e u t r a l TCNQ and TTF s t a c k s ( u s i n g t h e g e o m e t r y w h i c h t h e y have i n t h e m i x e d c r y s t a l (9)) and t o t h e s t a c k e d DNA b a s e s a p p l y i n g t h e same g e o m e t r y as i n a s i n g l e c h a i n o f t h e i n v i v o s t a b l e B form o f DNA ( 1 0 ) . The ST0-3G b a s i s h a s b e e n a p p l i e d a l s o f o r t h e s e c a l c u l a t i o n s and s i n c e t h e s t a c k i n g d i s t a n c e i n a l l t h e s e s y s t e m s i s o v e r 3& ( 3 . 4 7 & f o r T T F , 3 . 1 7 A f o r TCNQ and 3 . 3 6 $ f o r t h e h o m o p o l y n u c l e o t i d e s ) o n l y s e c o n d n e i g h b o u r ' s i n t e r a c t i o n s have b e e n t a k e n i n t o a c c o u n t . I n T a b l e I I we p r e s e n t t h e r e s u l t s f o r t h e v a l e n c e and c o n d u c t i o n bands o f t h e s e systems. I n t h e c a s e o f t h e h o m o p o l y n u c l e o t i d e s we g i v e a l s o ( i n parenthesis) v a l e n c e and c o n d u c t i o n bands c o r r e c t e d f o r l o n g r a n g e c o r r e l a t i o n u s i n g t h e e l e c t r o n i c p o l a r o n model ( 1 1 ) . I n t h e c a s e o f t h e TTF and TCNQ s t a c k s , i t i s i n t e r e s t i n g t o n o t e t h a t t h e v a l e n c e band o f TTF ( f r o m w h i c h i n t h e m i x e d c r y s t a l t h e c h a r g e t r a n s f e r o c c u r s ) and t h e c o n d u c t i o n band o f TCNQ ( t o w h i c h t h e c h a r g e i s t r a n s f e r r e d ) a r e c o m p a r a t i v e l y b r o a d ( ^ 0 . 3 eV and o» 1.2 e V , r e s p e c t i v e l y ) , w h i l e t h e c o n d u c t i o n band o f TTF and t h e v a l e n c e band o f TCNQ a r e v e r y n a r r o w . The p o s i t i o n o f t h e ?

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

76

PHOTON, ELECTRON, AND ION PROBES Table

II

The V a l e n c e and C o n d u c t i o n Bands o f TCNQ and TTF S t a c k s and o f Four Homopolynucleotides ( i n eV-s)

V a l e n c e Band

min TTF TCNQ Polycytosine Polythymine Polyad en i n e Polyguan i n e

-3.80 -7.25 -9.67 (-9.19) -9.58 (-9.22) -9.18 (-8.82) -8.03 (-7.65)

e

C o n d u c t i o n Band 6e

e

6e max

-3.50 -7.16 -9.11 (-8.67) -9.15 (-8.81 -8.7 (-8.41) -7.40 (-7.06)

the

min

max

0.30 0.09 0.56 (0.52) 0.43

8.51 -0.49 1.54 (1.12) 3.00

8.59 0.69 2.78 (2.27) 3.93

0.08 1.18 1.24 (1.15) 0.93

(0.41) 0.63 (0.59)

(2.48) 3.68 (3.35)

(3.41) 4.73 (4.36)

(0.93) 1.05 (1.01)

b a n d s ( h i g h - l y i n g v a l e n c e band o f TTF and l o w - l y i n g c o n d u c t i o n band o f TCNQ) f a v o r s a l s o s t r o n g l y the c h a r g e t r a n s f e r from TTF t o TCNQ. C o m p a r i n g the w i d t h s o f t h e v a l e n c e bands and o f t h e c o n d u c t i o n bands o f the f o u r h o m o p o l y n u c l e o t i d e s one f i n d s t h a t t h e y have a b o u t the same w i d t h s as the v a l e n c e band o f TTF and t h e c o n d u c t i o n band o f TCNQ. T h i s means t h a t i f one w o u l d dope them w i t h good e l e c t r o n a c c e p t o r s o r d o n o r s , one w o u l d e x p e c t a r a t h e r large c o n d u c t i v i t y a l s o i n these systems. One s h o u l d p o i n t o u t t h a t t h o u g h r e a l DNA i s a p e r i o d i c , t h e h o m o p o l y n u c l e o t i d e s ( w h i c h c o n t a i n a l s o the sugar phosphate p a r t n e g l e c t e d i n these c a l c u l a t i o n s , b u t w h i c h most p r o b a b l y w o u l d n o t e f f e c t t o o much t h e v a l e n c e and c o n d u c t i o n b a n d s , b e c a u s e b o t h a r e TT b a n d s ) have b e e n s y n t h e t i z e d i n the l a b o r a t o r y . A c c o r d i n g t o o u r k n o w l e d g e no t r a n s p o r t p r o p e r t y measurements o f doped h o m o p o l y n u c l e o t i d e s h a v e been p e r f o r m e d . The numbers i n p a r e n t h e s i s i n d i c a t e t h a t l o n g r a n g e c o r r e l a t i o n d e c r e a s e s t h e b a n d w i d t h s and gaps o n l y m o d e r a t e l y ( i n c o n t r a r y t o s i m p l e m e t a l s and i o n i c c r y s t a l s ( 1 1 ) ) . The A p e r i o d i c i t y P r o b l e m ; t h e (SN) - (SNH) S y s t e m . We have r e p o r t e d p r e v i o u s l y Q_2) an ab i n i t o S&F LCAO So band s t r u c t u r e c a l c u l a t i o n on the (SN) c h a i n u s i n g the e x p e r i m e n t a l g e o m e t r y ( 1 3 ) and a d o u b l e £ b a s i s s e t (_14). Though t h i s c a l c u l a t i o n t r e a t e d (SN) o n l y as a o n e - d i m e n s i o n a l s y s t e m , r a t h e r good agreement w i t h e x p e r i m e n t has been a c h i e v e d f g r the e f f e c t i v e mass and d e n s i t y o f s t a t e s a t the F e r m i l e v e l (m ( E ) = 1.71m , e x p : 2-Om^; p ( E ) = 0.17 ( e V s p i n mol) , exp:0.1*?) and w i t h ?he amount o f c h a r g e t r a n s f e r r e d from S t o N ( 0 . 4 e , e x p : 0 . 3 - 0 . 4 e ) . x

f

F

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

7.

LADIK ET AL.

11

Electronic Structure of Polymers

I n the l a s t y e a r s 4 - 8 m o l p e r c e n t h y d r o g e n i m p u r i t i e s have b e e n found i n (SN) a t I B M , San J o s e ( 1 5 ) . One o f the most p r o b a b l e s i t e o f H b o n d i n g i s the N atom i n the (SN) u n i t s . To i n v e s t i g a t e t h e e f f e c t o f r a n d o m l y d i s t r i b u t e d H atoms on t h e band structure o f (SN) we have p e r f o r m e d a s i n g l e s i t e one-band c o h e r e n t p o t e n t i a l a p p r o x i m a t i o n (CPA) c a l c u l a t i o n f o r the ( S N ) - (SN) m i x e d s y s t e m a s s u m i n g d i f f e r e n t c o n c e n t r a t i o n s o f h y d r o A gen i m p u r i t i e s (_16). F o r t h i s p u r p o s e we have e x e c u t e d b e s i d e s t h e (SN) c h a i n a l s o a band s t r u c t u r e c a l c u l a t i o n f o r an ( S N L x

x

x

k

chain. S i n c e t h e d e n s i t y o f s t a t e s c u r v e s o f t h e two s y s t e m s a r e v e r y d i f f e r e n t , no c o n s t a n t s e l f e n e r g y ( z ) c o u l d be a p p l i e d , b u t one had t o use a k and e n e r g y dependent £(k,E). An a p p r o p r i a t e f o r m a l i s m h a s b e e n w o r k e d o u t ( f o r t h e d e t a i l s see (16^)) and t h e c o u p l e d Dyso i t e r a t i v e way u n t i l s e l A c c o r d i n g to t h e r e s u l t s o b t a i n e d a l r e a d y a t 3 p e r c e n t H s p i k e s and d i p s o c c u r i n t h e d e n s i t y o f s t a t e s c u r v e o f t h e m i x e d s y s t e m . A t l a r g e r c o n c e n t r a t i o n s o f i m p u r i t i e s due t o c l u s t e r i n g effect o f t h e i m p u r i t y , a p a r t o f the gaps d i s a p p e a r (16). F i n a l l y , i t s h o u l d be m e n t i o n e d t h a t w i t h t h e i n c r e a s e o f t h e i m p u r i t y c o n c e n t r a t i o n , as t h e c a l c u l a t i o n s show, t h e d e n s i t y o f s t a t e s a t t h e F e r m i l e v e l i n c r e a s e s and t h e r e f o r e we w o u l d e x p e c t an i n c r e a s e o f t h e t r a n s i t i o n t e m p e r a t u r e b e t w e e n t h e s u p e r c o n d u c t i n g and n o r m a l s t a t e s . ( T h i s e x p e r i m e n t , as f a r as we know, has n o t been p e r f o r m e d y e t ) . We c a n c o n c l u d e from t h i s CPA c a l c u l a t i o n t h a t a p e r i o d i c i t y ( d i s o r d e r ) has a r a t h e r serious e f f e c t on t h e band s t r u c t u r e o f p o l y m e r s . T h i s f i n d i n g was s u p p o r t e d by an i n v e s t i g a t i o n o f t h e m i x e d g l y c i n e - a l a n i n e system (the c o r r e s p o n d i n g h o m o p o l y p e p t i d e s have v e r y s i m i l a r band s t r u c t u r e s ) t a k i n g 1 , 0 0 0 - 1 0 , 0 0 0 u n i t s o f t h e mixed c h a i n o f g i v e n sequences. I n the course o f t h i s study ( 1 7 ) , the H u c k e l m a t r i c e s o f the l o n g c h a i n s ( t h e d i a g o n a l elements o f t h e s e m a t r i c e s were f i x e d on t h e b a s i s o f t h e p o s i t i o n s o f t h e SCF LCAO bands o f t h e homogeneous c h a i n s and t h e o f f - d i a g o n a l 3. . p a r a m e t e r s have b e e n d e t e r m i n e d w i t h t h e h e l p o f t h e band w i d t h s ^ w e r e b r o u g h t t o an u p p e r t r i a n g u l a r form w i t h t h e a i d o f s u b sequent G a u s s i a n t r a n s f o r m a t i o n s . Using Dean's negative e i g e n v a l u e t h e o r e m (18) from t h e d i a g o n a l e l e m e n t s o f t h e t r a n s f o r m e d m a t r i x one c a n o b t a i n d i r e c t l y t h e d e n s i t y o f s t a t e s o f t h e m i x e d chains. The r e s u l t s w h i c h were o b t a i n e d show t h a t d e p e n d i n g on t h e s e q u e n c e and c o n c e n t r a t i o n o f t h e i m p u r i t y u n i t s ( a l a n i n e u n i t s i n a g l y c i n e c h a i n ) t h e band s t r u c t u r e o f p o l y g l y c i n e i s v e r y strongly effected. I f we p e r f o r m e d s e l f c o n s i s t e n t c a l c u l a t i o n s b e t w e e n the d i f f e r e n t u n i t s ( c l u s t e r c a l c u l a t i o n s t o t a k e i n t o account i n t e r u n i t s e l f c o n s i s t e n c y are i n p r o g r e s s ) probably t h i s e f f e c t w o u l d be s m a l l e r , b u t most p r o b a b l y s t i l l r a t h e r i m p o r t ant. Such c a l c u l a t i o n s are i n p r o g r e s s .

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

78

PHOTON, ELECTRON, AND ION PROBES

C o n c l u d i n g Remarks B e s i d e s the mentioned a p e r i o d i c i t y p r o b l e m the t r e a t m e n t o f c o r r e l a t i o n i n the g r o u n d s t a t e o f a p o l y m e r p r e s e n t s t h e most formidable problem. I f one has a p o l y m e r w i t h c o m p l e t e l y f i l l e d v a l e n c e and c o n d u c t i o n b a n d s , one c a n F o u r i e r t r a n s f o r m the d e l o c a l i z e d B l o c h o r b i t a l s i n t o l o c a l i z e d W a n n i e r f u n c t i o n s and u s e t h e s e ( i n s t e a d o f t h e MO-s o f t h e p o l y m e r u n i t s ) f o r a quantum c h e m i c a l t r e a t m e n t o f the s h o r t r a n g e c o r r e l a t i o n i n a s u b u n i t t a k i n g o n l y e x c i t a t i o n s i n the s u b u n i t o r b e t w e e n t h e reference u n i t and a few n e i g h b o u r i n g u n i t s . W i t h the a i d o f t h e W a n n i e r f u n c t i o n s t h e n one c a n p e r f o r m a M o e l l e r - P l e s s e t p e r t u r b a t i o n t h e o r y ( P T ) , or for i n s t a n c e , a coupled e l e c t r o n p a i r approximat i o n (CEPA) (18) or a c o u p l e d c l u s t e r e x p a n s i o n (19) c a l c u l a tion. The l o n g r a n g e c o r r e l a t i o n t h e n c a n be a p p r o x i m a t e d w i t h t h e h e l p o f the a l r e a d y m e n t i o n e d e l e c t r o n i c p o l a r o n m o d e l ( 1 1 ) The s i t u a t i o n i s muc f i l l e d valence band(s) o b t a i n l o c a l i z e d o r b i t a l s ( o n l y i f a commensurable p a r t , e.g. 1 / 2 , 1/4, e t c . p a r t o f the band i s f i l l e d w h i c h i s u s u a l l y n o t t h e c a s e ) , one u s u a l l y f a l l s b a c k t o homogeneous or inhomogeneous gas methods w h i c h a r e r a t h e r q u e s t i o n a b l e i n t h e c a s e o f o r g a n i c p o l y m e r s w i t h c o m p a r a t i v e l y s m a l l d e n s i t i e s and l a r g e density g r a d i e n t s ( i f there are h e t e r o atoms). A p r o m i s i n g p o s s i b i l i t y f o r the t r e a t m e n t o f c o r r e l a t i o n i n m e t a l l i c polymers i s to t r y to s u b d i v i d e the p a r t i a l l y f i l l e d band (and a few n e i g h b o u r i n g f i l l e d and u n f i l l e d b a n d s ) i n t o r e g i o n s each o f w h i c h i s c h a r a c t e r i z e d by a s i n g l e r e p r e s e n t a t i v e level ( t h i s d i s c r e t i z a t i o n o f a continuum i s f a m i l i a r i n the t h e o r y o f s c a t t e r i n g p r o c e s s e s , b u t has n o t been a p p l i e d f o r t h e c o r r e l a t i o n problem). T h i s s u b d i v i s i o n c a n be a c h i e v e d by s t u d y i n g t h e d e n s i t y o f s t a t e s c u r v e o f t h e band and i n v e s t i g a t i n g t h e change w i t h k o f the s p a t i a l d i s t r i b u t i o n o f t h e p a r t i a l c h a r g e due t o the band under c o n c e n t r a t i o n . ( U s i n g t h e s e two c r i t e r i a a d i v i s i o n o f t h e v a l e n c e and c o n d u c t i o n b a n d s o f p o l y c y t o s i n e has been a l r e a d y p e r f o r m e d (21)). A f t e r a c h i e v i n g a more or l e s s c o n s i s t e n t d i v i s i o n o f t h e b a n d , one c a n a t t r i b u t e t o e a c h r e g i o n a weight f a c t o r i n t e g r a t i n g the d e n s i t y o f s t a t e s curve over the g i v e n r e g i o n ( a n d t a k i n g the s q u a r e r o o t o f t h i s q u a n t i t y ) . One can t h e n p e r f o r m e x c i t a t i o n s b e t w e e n t h e d i s c r e t e l e v e l s r e p r e s e n t i n g t h e s e r e g i o n s and m u l t i p l y i n g e a c h m a t r i x e l e m e n t w i t h the c o r r e s p o n d i n g w e i g h t f a c t o r s . These q u a n t i t i e s c a n be s u b stituted f i n a l l y i n t o the e x p r e s s i o n s o f any s i z e consistent method f o r the t r e a t m e n t o f c o r r e l a t i o n ( M o e l l e r - P l e s s e t P . T . , C E P A , CPMET, e t c . ) . The i n v e s t i g a t i o n o f t h e method ( t h e d e p e n dence o f the r e s u l t s on the s u b d i v i s i o n o f t h e band) and i t s a p p l i c a t i o n to m e t a l l i c polymers i s i n p r o g r e s s . y

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

7.

LADIK ET AL.

79

Electronic Structure of Polymers

Acknovledgement s The a u t h o r s w o u l d like to express their gratitude to P r o f e s s o r s T . C . C o l l i n s , J . C i z e k , F . M a r t i n o and G . D e l Re f o r the many i n t e r e s t i n g s t i m u l a t i n g d i s c u s s i o n s . One o f us ( J . L . ) s h o u l d l i k e to a c k n o w l e d g e v e r y much the warm h o s p i t a l i t y o f P r o f e s s o r J . C i z e k and o f t h e D e p a r t m e n t o f A p p l i e d M a t h e m a t i c s , U n i v e r s i t y o f W a t e r l o o , extended to him d u r i n g h i s v i s i t in Waterloo.

Abstract For the calculation of the Hartree-Fock band structures of polymers a method has been developed including non-local exchange and full-self consistency. It is applicable also in the case of a combined symmetry (e.g., helix). The method has bee polydiacetylene chains and stacked). Applications have been done also to TCNQ and TTF stacks, to (SN) and to periodic DNA models (the four homopolynucleotides), to the sugar phosphate chain of DNA and to different periodic protein models (homopolypeptides). A l l these systems have relatively broad valence and conduction bands (bandwidths around or larger than 0.5eV) according to our results. In the final part of the lecture methods to treat the electronic correlation and the aperiodicity of polymers w i l l be outlined. For the latter case CPA results for a (SN) -(SN) mixed system w i l l be discussed and extensions to aperiodic | x

x

H

biopolymers (which are in progress) w i l l be indicated.

Literature Cited 1. Del Re, G., Ladik, J. and Biczo, G., Phys. Rev., 1967, 155, 997; Andre, J.-M., Gouverneur, L. and Leroy, G., Int. J. Quant. Chem., 1967, 1, 427 and 451. 2.

Blumen, A., Merkel, C., Phys. Stat Sol., 1977, B83, 425; Ladik, J., "Excited States in Quantum Chemistry," eds. C.A. Nicolaides and D.R. Beck, D. Reidel Publ. Co., DordrechtBoston-London, 1979, p. 495.

3.

Piela, L., Delhalle, J., Int. J. Quant. Chem., 1978, 13, 605; Andre, J.M., Fripiat, J.G., Demanet, C., Bredas, J.L. and Delhalle, J., Int. J. Quantum Chem., S12, (accepted); Suhai, S. and Seel, M., to be published.

4.

Itoh, T., Shirayawa, H. and Ikeda, S., J. Polymer Sci., Polymer Chem. Ed., 1974, 12, 11.

5.

Chance, R.R., Baughman, R.H., Muller, H. and Eckhardt, C.I., J. Chem. Phys., 1977, 67, 3616.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTON, ELECTRON, AND ION PROBES

6.

Hehre, W., Steward, R.F. and Pople, J.A., J. Chem. Phys., 1969, 51, 2657.

7.

Suhai, S., to be published.

8.

Suhai, S., to be published.

9.

Kistenmacher, T.J., Phillips, T.E. and Cowan, D.O., Acta Cryst., 1974, 33, 763.

10. Arnott, S., Dover, S.D. and Wonacott, A.J., Acta Cryst., 1969, B25, 2192. 11. Kunz, A.B., Phys. Rev., 1972, B6, 606; Collins, T.C., "Electronic Structure of Polymers and Molecular Crystals," J.-M. Andre and J. Ladik eds., Plenum Press, New York-London, 1975, p. 405. 12. Suhai, S. and Ladik, J., Solid State Comm., 1977, 22, 227. 13. Mikulski, C.M., Russo, P.J., Saran, M.S., Madoiarmid, A.G., Garito, A.F., Heeger, A.J., J. Am. Chem. Soc., 1975, 97, 6358. 14. Roos, B., Siegbahn, T., Theor. Chim. Acta, 1970, 17, 1209. 15. Smith, R.D., Wyatt, J.R., Weber, D., DeCorps, J.J., and Saalfed, F.E., Inorg. Chem., 1978, 17, 1639. 16. Seel, M., Collins, T.C., Martino, F., Rai, D.K. and Ladik, J., Phys. Rev., 1978, B18, 6460. 17. Seel, M., Chem. Phys., 1979, 43, 103. 18. Dean, P., Rev. Mat. Phys., 1972, 44, 127. 19. Meyer, W., J. Chem. Phys., 1972, 58, 1017; Kutzelnigg, W., J. Chem. Phys., 1975, 62, 1225. 20. Cizek, J., J. Chem. Phys., 1969, 36, 4256; Paldus, J. and Cizek, J., Adv. Quant. Chem., 1975, 9, 105. 21. Ladik, J., "Quantum Theory of Polymers," J.-M. Andre, J. Delhalle, J. Ladik and G. Leroy editors, Springer-Verlag, Berlin-Heidelberg-New York (in press). RECEIVED

December 22,

1980.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

8 Spectroscopic Studies of Polydiacetylenes D. B L O O R Department of Physics, Queen Mary College, Mile End Road, London E1 4NS, United Kingdom

C o n j u g a t e d p o l y m e r s have b e e n s t u d i e d f o r many y e a r s p r i n c i p a l l y as p o t e n t i a l s e m i c o n d u c t o r s . I n 1970 the f i e l d was seen t o be e x p a n d i n g , b u t s t i l l i n i t s e a r l y s t a g e s and l a c k i n g a u n i f y i n g theory (1). The a b s e n c e o f s u c h a t h e o r y was n o t due t o l a c k o f e f f o r t by t h e o r i s t s , who had d e v e l o p e d s e v e r a l a l t e r n a t i v e a p p r o a c h e s t o e x p l a i n t h e o c c u r r e n c e o f an e n e r g y - g a p i n c o n j u g a t e d polymers (2^9). The m a i n p r o b l e m was t h e amorphous s t r u c t u r e o f t h e c o n j u g a t e d p o l y m e r s t h e n a v a i l a b l e . T h u s , i t was n o t p o s s i b l e t o d i s t i n g u i s h i n t r i n s i c from e x t r i n s i c p r o p e r t i e s , f o r example t h e p a r a m a g n e t i s m o b s e r v e d i n a l l c o n j u g a t e d p o l y m e r s had b e e n a t t r i b u t e d to an i n t r i n s i c s p i n - a l t e r n a t i o n ( H ) ) , bond-altern a t i o n d e f e c t s (11) and t o a number o f o t h e r c a u s e s ( 1 2 ) . I n t e r e s t i n c o n j u g a t e d p o l y m e r s waned somewhat i n t h e e a r l y p a r t o f t h e l a s t decade b u t has r e - e m e r g e d f o l l o w i n g t h e d i s c o v e r y o f m e t a l l i c c o n d u c t i v i t y and s u p e r c o n d u c t i v i t y i n (SN) ( L 3 ) and m e t a l l i c c o n d u c t i v i t y i n doped (CH) ( 1 4 ) . The l a t t e r has b e e n shown n o t t o be u n i q u e ( L 5 , 16) i n d i c a t i n g t h e g e n e r a l i t y o f h i g h c o n d u c t i v i t y i n doped c o n j u g a t e d p o l y m e r s and t h e p o s s i b i l i t y o f the d i s c o v e r y o f t e c h n o l o g i c a l l y a p p l i c a b l e m a t e r i a l s . However, t h o u g h (CH) i s p a r t i a l l y c r y s t a l l i n e the m o r p h o l o g y i s still complex and t h e o b s e r v e d p r o p e r t i e s have b e e n i n t e r p r e t e d i n terms o f e i t h e r the f u n d a m e n t a l p r o p e r t i e s o f the (CH) p o l y m e r ( 1 4 , 1 7 ) , o r the d i s o r d e r e d and inhomogeneous n a t u r e o f t h e s a m p l e s (_18). T h u s , d e s p i t e the emergence o f sophisticated t h e o r i e s (17^ 2 9 , 20) t h e b a s i c p r o b l e m o f i n t e r p r e t a t i o n imposed by sample m o r p h o l o g y r e m a i n s . One a p p r o a c h to t h e d e t e r m i n a t i o n o f i n t r i n s i c p r o p e r t i e s , w h i c h has b e e n u t i l i z e d s i n c e the e a r l i e s t i n t e r e s t i n c o n j u g a t e d p o l y m e r s , i s t o s t u d y the p r o p e r t i e s o f r e l a t e d o l i g o m e r s , as i n the p r e c e d i n g paper ( 2 1 ) . I t i s , however, a l s o p o s s i b l e to study model m a c r o m o l e c u l e s , the p o l y d i a c e t y l e n e . The e x i s t e n c e o f s o l i d s t a t e p o l y m e r i z a t i o n i n d i a c e t y l e n e monomers has a l o n g h i s t o r y ( 2 2 , 2 3 , 2 4 , 2 5 ) , b u t i t was n o t t h o r o u g h l y s t u d i e d u n t i l 0097-6156/81/0162-0081 $06.00/ 0 © 1981 American Chemical Society In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTON, ELECTRON, AND ION PROBES

82

a b o u t t e n y e a r s ago ( 2 6 ) . I t was t h e n e s t a b l i s h e d t h a t the p o l y m e r i z a t i o n c o u l d p r o c e e d t o c o m p l e t i o n by a homogeneous s o l i d s t a t e r e a c t i o n t o g i v e a p o l y m e r s i n g l e c r y s t a l as t h e p r o d u c t (27, 28). The r e a c t i o n i s shown s c h e m a t i c a l l y i n F i g u r e 1, t y p i c a l e n d - g r o u p s ( R , R ) , w h i c h l e a d t o the f o r m a t i o n o f o r d e r e d p o l y m e r c h a i n s , a r e l i s t e d i n T a b l e I . As i n d i c a t e d i n t h e t a b l e numerous d i a c e t y l e n e compounds have now b e e n s y n t h e s i z e d , p o l y m e r i z e d and t h e i r p h y s i c a l p r o p e r t i e s s t u d i e d ; t h i s work has b e e n e x t e n s i v e l y r e v i e w e d ( 2 9 , 3 0 , 31_, 3 2 ) . The c r y s t a l s o b t a i n e d d i s p l a y the i n t r i n s i c p r o p e r t i e s o f the polymer s i n c e defect l e v e l s a r e s u f f i c i e n t l y low t o be a m i n o r p e r t u r b a t i o n . 1

Table

I

S u b s t i t u e n t groups f o r the p o l y d i a c e t y l e n e s which are t o i n t h e t e x t by the l i s t e d a b b r e v i a t i o n s

referred

R -CH OS0 C H CH 2

2

6

4

TS

Symmetric

3

-CH 0C0NHC.H.

PU

o

Z

0

3

-(CH ) OCONHC H

5

PUDD

-(CH ) OCONHC H

5

TCDU

-(CH ) OCONHC H

5

ETCD

2

3

2

6

4

2

6

4

2

-(CH ) OCONHCH(CH ) 2

4

3

-(CH ) 0

Z

n

0C0NHCH C00Z z

NZCMU

o

-CH NC H 2

IPUDO

2

1 2

DCH

8

-(CH ) OH 2

OD

2

"

-(CH ) OH 2

3

-COO(CH ) CH 2

g

DD CAP

3

-CH 0H

-CH

2

-(CH ) CH

3

-(CH ) C00H

-(CH ) CH

3

-(CH ) COO~.Li

2

2

9

9

2

2

-C H OCO(CH ) OCOC H 6

4

2

3

6

4

10H

3

TCDA

8

8

-(Cyclic)

+

Li-TCDA BPG

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Spectroscopic Studies of Polydiacetylenes

83

Jb-R

- c > C Q

R

X

R

Figure 1. Solid-state polymerization of diacetylenes shown schematically: (left) array of monomer molecules in the crystal lattice; fright; the resulting polydiacetylene chain. a

o_ * \ K

r u

n

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTON, ELECTRON, AND ION PROBES

84

The r a n g e o f e n v i r o n m e n t s t o w h i c h t h e p o l y m e r c h a i n s may be subject varies widely. I n the c r y s t a l l i n e s t a t e the s t r u c t u r e o f t h e l a t t i c e i s s t r o n g l y i n f l u e n c e d by the b u l k y s i d e - g r o u p s , w h i c h p r o v i d e t h e a r r a n g e m e n t o f monomer m o l e c u l e s n e c e s s a r y f o r s o l i d state r e a c t i v i t y . Thus, polymer m o l e c u l e s i n p a r t i a l l y or f u l l y p o l y m e r i z e d c r y s t a l s o f d i f f e r e n t monomers a r e s u b j e c t e d t o different static distortions. D r a m a t i c changes i n b o t h s t a t i c and dynamic d i s t o r t i o n s c a n o c c u r when t h e c r y s t a l l a t t i c e u n d e r goes a phase t r a n s i t i o n . Such changes c a n a l s o be p r o d u c e d when the polymer i s p r e p a r e d i n a d i s o r d e r e d form, e . g . i n s o l u t i o n (33), as c o l l o i d a l p a r t i c l e s ( 3 4 ) , by d e f o r m a t i o n o f s i n g l e c r y s t a l s ( 3 5 ) and by e x t r a c t i o n o f the p o l y m e r f r o m p a r t i a l l y polymerized c r y s t a l s (36). Studies of c r y s t a l s can, therefore, p r o v i d e i n f o r m a t i o n a b o u t f u n d a m e n t a l p r o p e r t i e s and t h e i r d e pendence on c h a i n s t r u c t u r e w h i l e the l e s s p e r f e c t samples c a n be u s e d t o s t u d y the m o d i f i c a t i o n o f p h y s i c a l p r o p e r t i e s by d i s order. These i d e a s w i l l b a l and ESR s p e c t r a o f a r a n g e o f p o l y d i a c e t y l e n e s . The i n d i v i d u a l m a t e r i a l s m e n t i o n e d have d i f f e r e n t s i d e - g r o u p s w h i c h a r e l i s t e d , t o g e t h e r w i t h the a b b r e v i a t i o n s used s u b s e q u e n t l y to i d e n t i f y the materials, i n Table I . Optical

Spectroscopy

The p o l a r i z e d , m e t a l l i c r e f l e c t i v i t y o f a f u l l y p o l y m e r i z e d p o l y d i a c e t y l e n e c r y s t a l shows i m m e d i a t e l y i t s i n t r i n s i c a n i s o tropy. A t y p i c a l r e f l e c t i o n s p e c t r u m , f o r p o l y TS (37^), i s shown i n F i g u r e 2 , t h e o p t i c a l c o n s t a n t s o b t a i n e d by K r a m e r s - K r o n i g a n a l y s i s show t h a t t h e a b s o r p t i o n maximum o c c u r s n e a r 2 eV ( 1 6 , 0 0 0 cm ) (38, 39). T h i s e l e c t r o n i c e x c i t a t i o n i s the o n l y s t r o n g f e a t u r e o f the s p e c t r u m o v e r t h e r a n g e 1.65 t o 10 eV ( 3 8 ) . The a b s o r p t i o n i s h i g h l y a n i s o t r o p i c w i t h an a b s o r p t i o n t e n s o r w i t h i t s p r i n c i p a l a x e s p a r a l l e l t o t h e p o l y m e r c h a i n and p e r p e n d i c u l a r t o the c h a i n , f i r s t i n the p l a n e o f the c h a i n and s e c o n d l y n o r m a l t o the p l a n e o f t h e c h a i n . The v a l u e s o f t h e . p r i n c i p a l a b s o r p t i o n c o e f f i c i e n t s f o r p o l y TS a t 2 eV a r e 9 x 10 , 2 x 10 and l e s s t h a n 5 cm . E s s e n t i a l l y s i m i l a r r e s u l t s have been o b t a i n e d f o r a number o f o t h e r p o l y m e r s , e . g . TCDU, ETCD, DCH, 4BCMU and IPUDO ( 4 0 , 4 1 , 4 2 , 4 3 ) . The t e m p e r a t u r e dependence o f t h e a b s o r p t i o n e n e r g y i s s m a l l , see F i g u r e 3 , as a c o n s e q u e n c e o f the v e r y s m a l l t h e r m a l e x p a n s i v i t i e s o f the c r y s t a l s ( 4 4 ) . The p o l y m e r s w i t h u r e t h a n e s i d e g r o u p s have c r y s t a l l o g r a p h i c p h a s e t r a n s i t i o n s i n v o l v i n g r e o r i e n t a t i o n o f t h e s i d e g r o u p s and r e l a t e d large s h i f t s i n absorption energy. F i l m s o f nBCMU p o l y m e r c a s t from s o l u t i o n have l a r g e c o n t i n u o u s s h i f t s i n a b s o r p t i o n e n e r g y (42). These i n d i c a t e the o n s e t o f c o n s i d e r a b l e d i s o r d e r a t h i g h t e m p e r t u r e s , as d i s c u s s e d l a t e r . The a b s o r p t i o n was i n i t i a l l y i n t e r p r e t e d as an i n t e r b a n d t r a n s i t i o n s i n c e the a b s o r p t i o n p r o f i l e was o f t h e c o r r e c t form f o r a v a n Hove s i n g u l a r i t y i n one d i m e n s i o n :

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

BLOOR

Spectroscopic Studies of Polydiacetylenes a

1.0-1

Physics Status Solidi

Figure 2. Reflection spectrum of poly TS crystal at room temperature for light polarized parallel to the polymer chains: (a) detail of the reflection in the visible and (b) spectrum over the range 1.6 to 12 eV (38).

DCH

0

100

200 300 Temperature [K]

400

Figure 3. Temperature dependence of the intense optical absorption for a number of polydiacetylenes. With the exception of the nBCMU solution cast films, all samples were crystalline.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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oc

2

(1)

(ha) - E ) g

where E i s the gap e n e r g y . H o w e v e r , t h e a b s e n c e o f marked p h o t o c o n d u c t i v i t y a t 2 eV ( 4 5 , 46) c a s t doubt on t h i s interpretation. The o r i g i n a l s u g g e s t i o n (_37) t h a t the a b s o r p t i o n i s due t o an e x c i t o n i s now w i d e l y a c c e p t e d . T h e o r e t i c a l models i n d i c a t e the e x i s t e n c e o f an e x c i t o n i c s t a t e w h i c h c a n s t e a l i n t e n s i t y from t h e i n t e r b a n d t r a n s i t i o n ( 4 7 , 4 8 , 4 9 ) . The l i n e shape c a n a l s o be d e s c r i b e d by an a s y m e t r i c L o r e n z t i a n ( 3 8 )

I(w)

=

r + A(W-O) )

c

o

(co-u) )

2

+

(2) r

2

o

where C i s a c o n s t a n t asymmetry p a r a m e t e r . Thi t o n s w i t h c o u p l i n g to low f r e q u e n c y phonon modes ( 5 0 ) . The p r e s e n c e o f s u c h l o w f r e q u e n c y s i d e b a n d s i n p o l y d i a c e t y l e n e s has been c o n f i r m e d by c o m p a r i s o n o f the a b s o r p t i o n p r o f i l e w i t h t h e p u r e e l e c t r o n i c p r o f i l e o b t a i n e d from r e s o n a n t Raman s c a t t e r i n g e x p e r i m e n t s (_51). The r e s u l t s o f the Raman e x p e r i m e n t s a r e a l s o c h a r a c t e r i s t i c o f an e x c i t o n i c s t a t e . Exciton surface p o l a r i t o n s have been o b s e r v e d i n p o l y TS ( 5 2 ) . Thus the w e i g h t o f e v i d e n c e i s i n f a v o r o f a s s i g n m e n t o f an e x c i t o n i c s t a t e t o t h e i n t e n s e optical absorption. H o w e v e r , as shown i n F i g u r e 3 the a b s o r p t i o n c a n o c c u r o v e r a w i d e e n e r g y r a n g e . A somewhat l a r g e r r a n g e o f a b s o r p t i o n e n e r g i e s i s found f o r p o l y m e r c h a i n s i n p a r t i a l l y p o l y m e r i z e d c r y s t a l s ( 5 3 ) , see F i g u r e 4 . T h i s , and the g e n e r a l l y l a r g e r t e m p e r a t u r e dependence, reflects the greater variation in environment p o s s i b l e i n p a r t i a l l y p o l y m e r i z e d c r y s t a l s as compared w i t h f u l l y polymerized c r y s t a l s . A t l o w t e m p e r a t u r e s t h e s m a l l changes i n l i n e w i d t h and p o s i t i o n c a n be d e s c r i b e d by c o u p l i n g w i t h a Debye phonon s p e c t r u m ( 5 4 ) and a t h i g h e r t e m p e r a t u r e s by t h e r e s p o n s e o f t h e p o l y m e r c h a i n s t o t h e monomer l a t t i c e e x p a n s i o n (51.). The parameters r e l a t i n g a b s o r p t i o n energy to polymer c h a i n deformat i o n were t a k e n from the r e s u l t s o b t a i n e d f o r p o l y TS s a m p l e s i n e x t e n s i o n ( 5 5 ) and u n d e r h y d r o s t a t i c p r e s s u r e ( 5 6 ) . This i n t e r p r e t a t i o n i s b a s e d on the o c c u r r e n c e o f p o l y m e r c h a i n s w i t h a l a r g e number o f r e p e a t u n i t s , o f the o r d e r 100 o r m o r e , even a t low l e v e l s o f c o n v e r s i o n t o p o l y m e r . E v i d e n c e s u p p o r t i n g t h i s i s p r o v i d e d by m o l e c u l a r w e i g h t measurements f o r r a d i a t i o n p o l y m e r i z e d samples ( 3 3 ) , and by the s m a l l o p t i c a l l i n e w i d t h s o b s e r v e d f o r p a r t i a l l y p o l y m e r i z e d samples at low t e m p e r a t u r e s which are c o m p a r a b l e w i t h t h o s e o f f u l l y p o l y m e r i z e d samples ( 3 8 ) . It s h o u l d be n o t e d t h a t the s h i f t s i n a b s o r p t i o n e n e r g y shown i n F i g u r e 4 c o u l d n o t r e a d i l y be p r o d u c e d by e x t e r n a l f o r c e s .

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

8.

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Spectroscopic Studies of Polydiacetylenes

87

The spectra of fully and partially polymerized polydiacetylenes can be described in terms of the response of the polymer chain to its environment. The most important contribution is taken to be the static (time averaged) deformation of the polymer backbone. The dynamic (thermal) deformations are considered to give rise to the homogeneous (temperature dependent) linewidths. Large inhomogeneous linewidths can also be observed in highly strained or disordered systems. The static deformation primarily depends on the side-groups, which determine the lattice packing of the monomer crystal and, therefore, to a large extent the polymer crystal structure. The influence of the side-groups w i l l be much less in solution but, until recently, there have been few studies of polydiacetylene solutions since most fully polymerized materials are insoluble. However, a number of soluble polydiacetylenes have now been identified (33, 34, 57, 58, 59, 60). In the case of nBCMU polymers hydrogen-bondin single polymer chain ca solution (57), which has an absorption energy close to that of single crystals. A l l other solutions have a broad absorption spectrum, as shown in Figure 5, which is independent of the sidegroup. The electronic origin of this absorption does not occur at the absorption maximum but, as shown by second derivative _and fluorescence spectra (34), lies near 2.5 eV (^20,000 cm ). Colloidal polymer powders have been obtained, which retain the solution conformation; these have a narrower linewidth with the electronic absorption at 2.5 eV (20,160 cm ), as shown in Figure 6. The interpretation of these results is based on the fact that the polymer chains retain an extended form in solution. This has been shown by viscosity measurements (33) and the observation of distinct spectral features for the colloidal polymer. If chain deformation primarily produced a distribution of short conjugation sequences, as has been suggested (57), this spectrum would have a featureless profile; the alternative of a single well defined fold length does not appear reasonable. The range of spectral profiles observed can be explained simply in terms of differences in linewidth. Figure 7 shows calculated profiles using the lineshape of Equation 2 and including three phonon sidebands, further details of these calculations are given elsewhere (34J. Linewidth factors of 100, 300,^800 and more than 1,200 cm give profiles similar to those of poly TS as single crystals of 4K and 300K, colloidal particles and in solution respectively. In the latter two cases the linewidths must also contain an inhomogeneous broadening factor, this has been simply treated as a contribution to the linewidth factor. These factors appear reasonable for the different environments involved. Thus, we conclude that even in solution the time averaged structure of the extended polymer backbone is the main factor determining the

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTON, ELECTRON, AND ION PROBES

Figure 4. Temperature dependence of the intense optical absorption for polydiacetylene chains in their monomer matrices at typical concentrations of 1%; all samples were crystalline.

100

100 300 Temperature [K]

400

0.15-1

Figure 5. Absorption spectrum of poly TS in chloroform. The rising absorption at 300 nm is due to side-group absorption, principally to the monomer, which was present at about 100 times the concentration of the polymer.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

8.

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Spectroscopic Studies of Polydiacetylenes

89

1.0-

TS colloidal powder precipitated from chloroform solution. Similar results are obtained for acetone and nitrobenzene

Wavelength [nm]

0

2

Wavenumber [10 cm ] 3

1

Figure 7. Model absorption profiles calculated from Equation 2 with A = 0.08 and vibrational sideband frequencies of 950, 1480, 2080 cm' . The integrated intensities are not normalized, resulting in a vertical displacement as the linewidth parameter takes the values 100, 300, 500, 1,000, and 2,000 cm' , respectively. 1

1

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTON, ELECTRON, AND ION PROBES

a b s o r p t i o n e n e r g y , w h i l e the dynamic p e r t u r b a t i o n s d e t e r m i n e t h e linewidth. The i n f l u e n c e o f t h e s i d e - g r o u p s c a n a l s o be r e d u c e d i n the s o l i d s t a t e . I f t h e c r y s t a l s t r u c t u r e becomes d i s o r d e r e d t h e r e i s a g e n e r a l t e n d e n c y f o r the a b s o r p t i o n e n e r g y t o i n c r e a s e . This may o c c u r i n t h e s o l i d e i t h e r due t o a phase t r a n s i t i o n , for example i n the h i g h temperature d i s o r d e r e d phases o f L i - T C D S (60) as i n d i c a t e d i n F i g u r e 4 , o r by e x t r a c t i o n o f the p o l y m e r from t h e monomer l a t t i c e a t low c o n v e r s i o n or a c o m b i n a t i o n o f b o t h , a s f o r nBCMU f i l m s , F i g u r e 3 . For e x t r a c t e d polymer unless there i s s t r o n g s i d e - g r o u p i n t e r a c t i o n , t h e a b s o r p t i o n e n e r g y moves t o wards t h e s o l u t i o n v a l u e (_59). Typical absorption energies for e x t r a c t e d j i o l y m e r s a r e 2 . 2 6 eV ( 1 8 , 2 5 0 cm_ ) f o r OD, 2 . 2 9 eV ( 1 8 , 4 2 0 cm ) f o r DD and 2 . 3 1 eV ( 1 8 , 6 2 0 cm ) f o r TCDA. These r e s u l t s show t h a t the a b s o r p t i o n e n e r g y o f a p o l y d i a c e t y l e n e e x t e n d e d c h a i n s i n s o l u t i o n i s 2 . 5 eV and t h a t t h e lower v a l u e s observed fo r i g i d - p l a n a r conformatio p o s e d on the p o l y m e r by the i n t r a - and i n t e r - m o l e c u l a r i n t e r a c t i o n s o f the s i d e - g r o u p s . W h i l e r e a s o n a b l e agreement h a s b e e n o b t a i n e d i n some c a s e s , u s i n g e x p e r i m e n t a l l y d e t e r m i n e d p a r a m e t e r s , the d e t a i l s o f the r e l a t i o n s h i p between a b s o r p t i o n energy and s t r u c t u r e have y e t to be worked o u t . P r o g r e s s i n t h i s d i r e c t i o n i s h i n d e r e d by t h e l i m i t e d number o f polymer s t r u c t u r e s a v a i l a b l e . I n a d d i t i o n , the l a r g e s t s h i f t s i n e x c i t a t i o n e n e r g y o c c u r i n s i t u a t i o n s where X - r a y s t r u c t u r a l i n f o r m a t i o n i s not r e a d i l y o b t a i n e d , i . e . i n d i s o r d e r e d polymers and p a r t i a l l y p o l y m e r i z e d c r y s t a l s ( s e e F i g u r e s 3 and 4 ) . Even when s t r u c t u r a l i n f o r m a t i o n i s a v a i l a b l e i t may be inadequate, e . g . the s p l i t t i n g o b s e r v e d f o r TS ( F i g u r e s 3 and 4 ) o c c u r s f o r d i s t o r t i o n s w h i c h a r e n o t d e t e c t e d by s t r u c t u r a l a n a l y s i s . I n t h e c a s e o f TCDU t h e X - r a y s t r u c t u r e shows a p o l y b u t a t r i e n e c h a i n , RC = C = C = C R , w h i c h has p r o m p t e d t h e a s s i g n m e n t o f a h i g h a b s o r p t i o n e n e r g y to t h i s s t r u c t u r e ( 4 0 , 4 1 , 4 2 , 4 3 ) . However, the s t r u c t u r e shows a number o f o t h e r u n u s u a l bond l e n g t h s ( 6 1 ) . T h i s s u g g e s t s a f o r c e d computer f i t o f a s l i g h t l y d i s o r d e r e d s t r u c t u r e , an e f f e c t w h i c h has b e e n o b s e r v e d f o r o t h e r p o l y d i a c e tylenes (62). Thus, i n view o f the e s s e n t i a l l y c o n t i n u o u s spread o f e x p e r i m e n t a l a b s o r p t i o n e n e r g i e s o v e r t h e r a n g e 1.6 t o 2 . 5 e V , t h i s a s s i g n m e n t i s open t o q u e s t i o n . F u r t h e r d o u b t i s c a u s e d by the f a c t t h a t the p o l y b u t a t r i e n e c h a i n s h o u l d have a somewhat l o w e r e x c i t a t i o n e n e r g y t h a n the y n e - e n e s t r u c t u r e o f F i g u r e 1 (51). I n a d d i t i o n r e c e n t c a l c u l a t i o n s s u g g e s t t h a t the p o l y b u t a t r i e n e s t r u c t u r e does n o t have a s t a b l e e n e r g y minimum, i n t h e absence o f side-group i n t e r a c t i o n s ( 6 3 ) . Raman s t u d i e s o f c o l l o i d a l p o l y m e r p a r t i c l e s show t h a t the d i s o r d e r e d c h a i n s have bond l e n g t h s c l o s e to an i d e a l y n e - e n e s t r u c t u r e (34). This s u g g e s t s t h a t a more d e t a i l e d a n a l y s i s o f the e x c i t a t i o n e n e r g i e s o f deformed y n e - e n e c h a i n s i s c a p a b l e o f e x p l a i n i n g t h e e x p e r i mental data.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

8.

BLOOR

Spectroscopic Studies of Polydiacetylenes

E l e c t r o n S p i n Resonance

91

Spectroscopy

The existence i n conjugated polymers of paramagnetic c e n t e r s , w i t h g v a l u e s c l o s e t o t h e f r e e e l e c t r o n v a l u e and h a v i n g a n a r r o w s p e c t r a l l i n e w i d t h , h a s b e e n known f o r many y e a r s ( 6 4 , 65). I t w a s , t h e r e f o r e , n a t u r a l t o i n v e s t i g a t e t h e ESR s p e c t r u m o f p o l y d i a c e t y l e n e s . The e a r l i e s t s t u d i e s r e v e a l e d a s i m i l a r b u t v e r y weak ESR s i g n a l ( 3 7 , 6 6 ) . More d e t a i l e d s t u d i e s r e v e a l e d t h a t t h e s i g n a l s t r e n g t h was c o r r e l a t e d w i t h sample h i s t o r y and was v e r y weak i n t h e b e s t s i n g l e - c r y s t a l samples (j>7, 6 8 , 6 9 ) . I t was, t h e r e f o r e , c o n c l u d e d t h a t t h e p a r a m a g n e t i c c e n t e r s i n p o l y d i a c e t y l e n e s were a s s o c i a t e d w i t h d e f e c t s a n d , u n l i k e t h e o p t i c a l a b s o r p t i o n d i s c u s s e d a b o v e , were n o t an i n t r i n s i c p r o p e r t y o f t h e i d e a l polymer c h a i n . However, the s p i n c o n c e n t r a t i o n f o r d i s o r d e r e d samples was o n l y an o r d e r o f m a g n i t u d e l a r g e r t h a n t h a t i n good s i n g l e c r y s t a l s ( 6 9 ) s h o w i n g t h a t o n l a small f r a c t i o n of a l l t h e d e f e c t s were p a r a m a g n e t i c Subsequent s t u d i e s primarily f i c a t i o n of r e a c t i v e species present during p o l y m e r i z a t i o n (68, 70-74). The emergence o f (CH) , t h e r e d i s c o v e r y o f i t s ESR s p e c t r u m ( 7 5 ) and t h e i n t e r p r e t a t i o n u s i n g t h e s o l i t o n m o d e l ( 1 7 ) c a u s e d renewed i n t e r e s t i n the paramagnetic c e n t e r s in fully polymerized polydiacetylenes. I n i t i a l s t u d i e s were c o n c e r n e d w i t h the p o s s i b i l i t y o f doping p o l y d i a c e t y l e n e samples. In g e n e r a l t h i s was u n s u c c e s s f u l b e c a u s e e i t h e r t h e dopant c o u l d n o t d i f f u s e i n t o the s i n g l e c r y s t a l samples or i t r e a c t e d w i t h the side-groups. H o w e v e r , e x t r a c t e d p o l y 10H samples were f o u n d t o absorb l a r g e q u a n t i t i e s o f i o d i n e w i t h a consequent l a r g e i n c r e a s e i n t h e g ^ 2 ESR s i g n a l ( 7 6 , 7 7 ) . The b r o a d s i g n a l i n doped s a m p l e s c a n be a t t r i b u t e d t o r a d i c a l s formed a t s t a t i c defects, b u t on i l l u m i n a t i o n w i t h v i s i b l e l i g h t a n a r r o w s p e c t r a l compone n t , s i m i l a r t o t h e s t a b l e found i n (CH) , a p p e a r e d i n b o t h doped and undoped s a m p l e s . The s t r u c t u r e o f 10H monomer i s shown i n F i g u r e 8 ( 7 8 ) . A d j a c e n t rows o f monomer a r e l i n k e d by h y d r o g e n bonds and t h o u g h t h e p a c k i n g i s c l o s e t o t h e optimum f o r s o l i d s t a t e r e a c t i v i t y f a i l u r e o f t h e w e a k e r V a n d e r W a a l s bonds l e a d s t o b r e a k up o f t h e c r y s t a l before p o l y m e r i z a t i o n i s complete. Small q u a n t i t i e s o f f i b r o u s p o l y m e r w i t h a g r e e n i s h l u s t e r c a n be o b t a i n e d by d i s s o l v i n g t h e monomer l a t t i c e a t l ow l e v e l s o f c o n v e r s i o n . These samples g i v e a w e l l d e f i n e d powder X - r a y d i f f r a c t i o n p a t t e r n i n d i c a t i n g a h i g h degree o f c r y s t a l l i n i t y . E l e c t r o n micrographs show f i n e f i b e r s , up t o 25 nm i n d i a m e t e r , e x t e n d e d i n t h e p o l y m e r c h a i n d i r e c t i o n w i t h f i b e r b u n d l e s w i t h l e n g t h s g r e a t e r t h a n 20ym (79). The g ^ 2 E S R ^ s i g n a l o f s u c h s a m p l e s has a l i n e w i d t h o f a b o u t 10 g a u s s ( 1 0 T ) and i s o f l o w i n t e n s i t y . This broad component i n c r e a s e s i n i n t e n s i t y on i o d i n e d o p i n g . When an undoped sample i s i l l u m i n a t e d an a d d i t i o n a l n a r r o w component, w i t h i n t e n s i t y r e l a t e d to the i r r a d i a t i o n i n t e n s i t y , i s observed. T y p i c a l s p e c t r a taken at d i f f e r e n t temperatures a r e shown i n Figure 9 to 11.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Figure 8. Crystal structure of 10H monomer projected on to the ab-plane. Hydrogen bonds between H(01) and 0(1/ are shown dashed. Polymer chains grow with their axes along the chain lines.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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The n a r r o w s p e c t r a l component was found t o be m e t a s t a b l e down t o 4 . 2 K . The i n c r e a s e o f s i g n a l under i l l u m i n a t i o n and t h e s u b s e q u e n t decay was r e p r o d u c i b l e f o r r e p e a t e d i l l u m i n a t i o n . The r a t e o f g r o w t h and decay was t e m p e r a t u r e d e p e n d e n t , as shown i n F i g u r e 1 2 . These c u r v e s c a n n o t b e d e s c r i b e d by a s i n g l e e x p o n e n t i a l f a c t o r a n d , a l t h o u g h t h e d a t a i s i n s u f f i c i e n t f o r an a c c u r a t e A r h e n i u s p l o t , the a c t i v a t i o n energy appears t o be l o w , _7> 12.)S p e c i f i c examples i n c l u d e Sg ( 6 ) , TCNQ ( 6 ) , a n t h r a c e n e ( 8 ) , e t h y l b e n z e n e ( 2) = 1.5 ±0.1 eV. po

i 16

1

—i—•—i—«—i— —i— —i 1

14 12 10 8 BINDING ENERGY (eV)

1

6

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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i n g e n e r g y ) s i d e f o r n e a r n o r m a l e l e c t r o n e x i t ( 8 = 20 ) . This broadening i s not observed i n o t h e r w i s e i d e n t i c a l s t u d i e s employi n g l a r g e r c a g e l i k e m o l e c u l e s where m o l e c u l a r d i m e n s i o n s are l a r g e r t h a n the e l e c t r o n e l a s t i c mean f r e e p a t h , X , and t h u s t h e e f f e c t i s not expected to o c c u r (Ij*). The d a t a o f F i g . 2 a r e n o r m a l i z e d t o a p p r o x i m a t e l y e q u a l peak i n t e n s i t y f o r d i s p l a y purposes. The dashed l i n e f o r the 0 = 80 data ( e l e c t r o n e x i t n e a r l y p a r a l l e l to the s u r f a c e ) are e x p e c t e d to c o r r e s p o n d to o n l y t h e t o p m o l e c u l a r m o n o l a y e r when X ^ d , t h e m o l e c u l a r t h i c k n e s s (38). A s an e s t i m a t e o f X f o r the p r e s e n t s t u d y , r e c a l l t h a t N i e l s e n has d e t e r m i n e d t h a t X i s a p p r o x i m a t e l y one m o l e c u l a r t h i c k n e s s ( a b o u t 3 . 5 X) i n t e t r a t h i a f u l v a l e n e ( T T F ) and t e t r a c y a n o q u i n o d i m e t h a n e (TCNQ) m o l e c u l e s a t a b o u t 5-eV e l e c t r o n k i n e t i c e n e r g y above t h e vacuum l e v e l ( 3 1 ) . T h u s , u s i n g the l e a d i n g ( l o w - b i n d i n g - e n e r g y ) edge o f the spectrum to determine the l i n e shape t h e peak shown as a d a s h e d l i n e i n F i g . 2 represent anthracene. The f u l l w i d t eV. T h i s w i d t h i s n o t due t o sample c h a r g i n g i n v i e w o f t h e 2 0 - A mean t h i c k n e s s and t h e s h a r p n e s s o f t h e o b s e r v e d c u t o f f o f t h e H e l secondary-electron spectrum. O r i g i n s o f t h i s 0 . 7 eV c o n t r i b u t i o n to t h e UPS l i n e w i d t h w i l l be d i s c u s s e d b e l o w . Note t h a t the r e l a x a t i o n - e n e r g y s h i f t , E , i s 1.2 eV f o r I P ( 1 ) , i n agreement w i t h the r e s u l t s o f Grobman and K o c h (_11) f o r a p p r o x i m a t e l y t a n g e n t i a l e l e c t r o n e x i t . H e r e E i s r e f e r r e d t o t h e most i n t e n s e peak i n v i b r a t i o n a l l y s p l i t I P ( l f c o n s i s t e n t w i t h R e f . 11 r a t h e r t h a n the c e n t e r o f g r a v i t y o f the v i b r a t i o n a l e n v e l o p e . This l a t t e r c a s e w o u l d l e a d t o an i n c r e a s e i n t h e m a g n i t u d e o f E by about 0.1 eV. F i n a l l y , a t 6 = 80 , t h e u n s c a t t e r e d e l e c t r o n s emanate from X ( 8 0 ° ) = X ( c o s 8 0 ° ) = 0 . 1 7 X f r o m w i t h i n the s o l i d . S i n c e the i n t e r m o l e c u l a r spacing i n anthracene i s (8.7 X per u n i t c e l l ) / ( 3 m o l e c u l e s per u n i t c e l l ) = 2 . 9 X/molecule ( 4 0 ) , X would have to b e a b o u t 26 X i n o r d e r t o see a s i g n i f i c a n t c o n t r i b u t i o n t o 1 P ( 1 ) f r o m the m i d d l e o f the s e c o n d a n t h r a c e n e l a y e r i n t h e 9 = 80 d a t a , an u n r e a s o n a b l e number a t 15 eV k i n e t i c e n e r g y (31., 3 9 ) . A l t e r n a t i v e l y , i f X =26 X, t h e n the f e a t u r e s o f t h e A u s u b s t r a t e w o u l d show d i s t i n c t i v e l y i n t h e 6 = 2 0 ° UPS d a t a f o r 20-X f i l m s . Thus t h e XPS t h i c k n e s s d e t e r m i n a t i o n a l o n e p u t s an u p p e r l i m i t on X i n t h e UPS e n e r g y r e g i o n o f l e s s t h a n 10 X. The 0 = 2 0 ° UPS d a t a o f F i g . 2 t h e n i n c l u d e c o n t r i b u t i o n s t o I P ( 1 ) from t h e s u c c e e d i n g s u b l a y e r s o f t h e s a m p l e . Using the d a s h e d l i n e as a measure o f I P ( 1 ) from o n l y t h e f i r s t m o l e c u l a r m o n o l a y e r , t h e 0 = 20 d a t a were d e c o n v o l u t e d w i t h a Du P o n t N o . 610 c u r v e r e s o l v e r i n t o o n l y two peaks o f FWHM = 0 . 7 e V , o f a p p r o x i m a t e l y e q u a l i n t e n s i t y , and s e p a r a t e d i n e n e r g y by 0 . 3 e V . This fact i m p l i e s t h a t t h e s e c o n d and s u b s e q u e n t anthracene l a y e r s have E = 1.5 * 0 . 1 eV t o w i t h i n the a c c u r a c y o f t h i s experiment. A v e r y s i m p l e a n a l y s i s b a s e d upon t h e p e a k - i n t e n s i t y r a t i o and i n t e r m o l e c u l a r s p a c i n g y i e l d s a measure o f t h e e l a s t i c r

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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mean f r e e p a t h f o r t h e p h o t o e l e c t r o n s i n v o l v e d : X = 4 X a t 15 eV above t h e vacuum l e v e l , c o n s i s t e n t w i t h t h e 20-X s a m p l e t h i c k n e s s n e c e s s a r y to o b s c u r e a l l t r a c e s o f p h o t o e m i s s i o n from t h e A u s u b s t r a t e i n t h e UPS d a t a , w h i l e n o t i n t h e XPS d a t a , where * = 22 X a t a b o u t 1200 eV ( 3 6 ) . The s u r f a c e e f f e c t d i s c u s s e d above i s an a s p e c t o f i n t e r molecular r e l a x a t i o n i n molecular s o l i d s i s that associated with t h e d e p t h o f the m o l e c u l e from t h e sample s u r f a c e . This effect m a n i f e s t s i t s e l f as an a p p a r e n t l i n e - b r o a d e n i n g i n UPS s p e c t r a , o t h e r c o n t r i b u t i o n s to l i n e - b r o a d e n i n g a l s o e x i s t . T h e s e , as w e l l as some s p e c i f i c mechanisms t h a t l e a d t o t h e observed i n t e r m o l e c u l a r p o l a r i z a t i o n e f f e c t w i l l be d i s c u s s e d b e l o w . Homogeneous and Inhomogeneous C o n t r i b u t i o n s . Two o t h e r c o n t r i b u t i o n s to UPS l i n e w i d t h s f o r m o l e c u l a r s o l i d s have been a r t i c u l a t e d i n a study o f i s o p r o p y l benzene f i l m s at low tempera t u r e s ( 2 4 ) . The shape p r o h i b i t e d the e x p l i c i cussed for anthracene. I s o p r o p y l b e n z e n e was o f i n t e r e s t as a model m o l e c u l e f o r p o l y s t y r e n e , h o w e v e r . The measurements were c a r r i e d o u t on c o n d e n s e d m o l e c u l a r - s o l i d f i l m s i n t h e t e m p e r a t u r e r a n g e 15°K < T < 1 5 0 ° K . I s o p r o p y l benzene, a schematic diagram o f which i s i n c l u d e d i n F i g . 3 , was u t i l i z e d i n t h i s s t u d y f o r s e v e r a l r e a s o n s . The p r i m a r y m o t i v i a t i o n f o r the s t u d y was the a c q u i s i t i o n o f d a t a on the temperature dependence o f UPS l i n e w i d t h s i n m o l e c u l a r solids. These d a t a m i g h t t h e n e i t h e r c o n f i r m o r d i s p r o v e o u r r e c e n t l y - p r o p o s e d d i e l e c t r i c model o f c h a r g e - i n d u c e d r e l a x a t i o n i n t h e s e m a t e r i a l s ( 2 0 , 21)' S i n c e p o l y s t y r e n e ( P S ) and c o n d e n s e d f i l m s o f e t h y l b e n z e n e had b e e n s t u d i e d as p r o t o t y p e s y s t e m s i n an e a r l i e r i n v e s t i g a t i o n o f the r e l a x a t i o n e n e r g y s h i f t s i n s o l i d s t a t e v e r s u s gas phase p h o t o e m i s s i o n s p e c t r a ( 2 0 ) , i t was a p p r o p r i a t e to u t i l i z e an a n a l o g o u s model m o l e c u l e i n t h e present e x a m i n a t i o n o f the f l u c t u a t i o n - i n d u c e d w i d t h s o f the s o l i d - s t a t e spectra. I s o p r o p y l b e n z e n e has a l o w e r v a p o r p r e s s u r e than benzene and e t h y l b e n z e n e w h i c h were s t u d i e d p r e v i o u s l y ( 2 0 ) , c o n s e q u e n t l y , i t lends i t s e l f to i n v e s t i g a t i o n over a s l i g h t l y l a r g e r temperature range. I s o p r o p y l b e n z e n e i s o t h e r w i s e as s u i t a b l e a m o d e l m o l e c u l e f o r p o l y s t y r e n e as e t h y l b e n z e n e . The UPS s p e c t r u m , o b t a i n e d w i t h 32 eV p h o t o n s , f o r a 15 X t h i c k v a p o r - d e p o s i t e d f i l m o f i s o p r o p y l b e n z e n e 30 K i s shown i n F i g . 3. A l s o shown i n t h e f i g u r e i s the UPS s p e c t r u m from the c l e a n p a l l a d i u m c r y s t a l u s e d as a s u b s t r a t e s i g n a l , and the i s o p r o p y l b e n z e n e UPS s p e c t r a p r e v i o u s l y o b t a i n e d i n the gas p h a s e a t 2 1 . 2 eV p h o t o n e n e r g y ( 4 1 ) . The d a t a on the t e m p e r a t u r e dependence o f t h e width* o f the lowest b i n d i n g energy i o n i z a t i o n ( i . e . , at a b i n d i n g energy o f a b o u t 7.6 eV) from t h i c k e r , a p p r o x i m a t e l y 25 X f i l m s o f i s o p r o p y l benzene a r e summarized i n F i g . 4 . The d a t a have been a n a l y z e d e x p e r i m e n t a l l y by a s s u m i n g t h a t the o b s e r v e d f u l l - w i d t h - a t - h a l f -

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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I i i i i I i i i

20

15

i I i i i

10

i I

5

BINDING ENERGY (eV) FOR SOLIDS CURVES

Figure 3.

UPS spectra of the clean Pd substrate of a molecular thin-film (~ 15 A) sample of isopropyl benzene in the gas phase (41). The photon energies are 32 eV, 32 eV, and 21.2 eV (top to bottom). The energy scale is for the solid-state spectra and should be shifted to read higher building energy by 1.5 eV for the gas-phase spectrum. The isopropyl benzene film in this case is only about 15-A thick, and consequently the Pd d-band is not totally obliterated. The data of Figure 4 are for approximately 25-A thick films, where the Pd signal can not be seen.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SALANECK

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Aromatic Pendant Group Polymers

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100

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150

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200

T(°K)

Figure 4. Square of the experimental temperature contribution to UPS linewidth, A(T) , as a function of temperature for two totally independent runs ( A , Q ) in the top panel. The large error bars reflect the signal-to-noise ratio relative to the magnitude of the to observed linewidths. In the lower panel, a solid line is drawn for the standard deviat of a gaussian linewidth using the slope from the upper panel, where c = 2.6 (±0.5) X lQ-zji/2 ( j jh n idth data points plotted in the lower panel show consistency with the T behavior predicted by a molecular-ion model calculation (20, 21). 2

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In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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maximum i s g i v e n by t h e s q u a r e r o o t o f t h e sum o f t h e s q u a r e s o f the i n d i v i d u a l c o n t r i b u t i o n s to the l i n e w i d t h , namely (T) = ( E i ? ) i

1

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,

(5)

where the s e t o f A . a r e i n d e p e n d e n t c o n t r i b u t i o n s t o 6 ( T ) , t h e observed peakwidth: i s t h e 0 . 6 7 eV l i n e w i d t h o f t h e peak i n t h e gas p h a s e ; A i s the i n s t r u m e n t a l r e s o l u t i o n e s t i m a t e d at 0.1 eV from t h e w i d t h o f t h e F e r m i l e v e l o f t h e c l e a n m e t a l s u b s t r a t e ; A = A ( T ) i s t h e T - d e p e n d e n t c o n t r i b u t i o n t h a t i s e x t r a c t e d from the data; and A i s the r e s i d u a l T - i n d e p e n d e n t contribution o b t a i n e d by t h e e x t r a p o l a t i n g t o T = 0 . O p e r a t i o n a l l y , ^re p l g t 6 ( T ) v e r s u s T , e x t r a p o l a t e t o T = 0 and t h e n s u b t r a c t ( A + A ) = 0 . 6 8 eV i n o r d e r t o d e t e r m i n e A . We f i n d A ^^).4 ev, d e p e n d i n g on t h e v a p o r d e p o s i t i o m u l t i p l e exposures to H o w e v e r , a f a s t 20 L e x p o s u r e y i e l d e d A % 0 . 6 e V . T h i s l a t t e r c a s e p r o b a b l y c o r r e s p o n d s to a g r e a t e r cPegree o f m o l e c u l a r d i s o r d e r i n the r e s u l t a n t sample. We have a t t r i b u t e d the r e s i d u a l , T-independent f a c t o r to s p a t i a l v a r i a t i o n s i n l o c a l s i t e e n e r g i e s and t o t h e c o r r e s p o n d i n g inhomogeneous i n t e r m o l e c u l a r r e l a x a t i o n ( p o l a r i z a t i o n ) e n e r g i e s as d e s c r i b e d p r e v i o u s l y (18^, 2 0 , 2 1 , 2 8 ) . A f t e r d e t e r m i n i n g the s p e c i f i c A c o n t r i b u t i o n f o r each s e p a r a t e r u n , the d a t a a r e p l o t t e d as A(T3 = 6(T) 6(T=0) , i n F i g . 4 f o r two t o t a l l y i n d e p e n d e n t r u n s . The d a t a c a n be c o n v e r t e d t o the s t a n d a r d d e v i a t i o n p a r a m e t e r o f a g a u s s i a n c u r v e , o , and p l o t t e d as a f u n c t i o n o f T , as a l s o shown i n F i g . 4 . Then a c u r v e t h a t goes as T a p p e a r s t o be c o n s i s t e n t w i t h t h e d a t a as shown in F i g . 4b. 1

q

A most i m p o r t a n t a s p e c t o f t h e s e d a t a i s t h e t e m p e r a t u r e dependence o f t h e w i d t h o f t h e m o l e c u l a r s o l i ^ i p h o ^ t o - i p ^ n i z a t i o n peak. T h i s T - d e p e n d e n c e i s o f t h e form A(T) = A + C T , which i s t h e form w h i c h had b e e n p r e v i o u s l y d i s c u s s e d , b u t n o t q u a n t i t a t i v e l y o b s e r v e d , i n c o n n e c t i o n w i t h an X - r a y p h o t o e l e c t r o n s p e c t r o s c o p y ( X P S ) s t u d y o f c o r e - l e v e l p e a k s o f the a l k a l i h a l i d e s (25). Sample c h a r g i n g ^ e f f e e t s i n h i b i t e d t h e s t u d y o f t h e l i n e a r T - d e p e n d e n c e o f A^ ( T ) i n t h e XPS c a s e . A l i n e a r temperature dependence o f A i s p r e d i c t e d by any m o d e l i n w h i c h a l o w f r e q u e n c y v i b r a t i o n a l mode, tfu) < k T , i s s t r o n g l y c o u p l e d t o t h e m o l e c u l a r i o n s t a t e ( 2 ^ , 25, 26, 27, 42, 43, 4 9 ) . The most o b v i o u s such c o u p l i n g i s a s s o c i a t e d the e x c i t a t i o n o f i n t r a m o l e c u l a r modes upon p h o t o i o n i z a t i o n ; a phenomenon w h i c h p r o d u c e s s t r u c t u r e on p h o t o i o n i z a t i o n l i n e s i n the gas phase as w e l l as solid state. E v a l u a t i o n o f the r e l e v a n t c o u p l i n g c o n s t a n t s f o r benzene b a s e d m o l e c u l a r i o n s ( 4 3 ) s u g g e s t s t h a t t h e c o u p l i n g t o t h e s e modes i s t o o wea^ t o p r o v i d e a q u a n t i t a t i v e d e s c r i p t i o n o f t h e o b s e r v e d v a l u e o f c , a l t h o u g h we c a n n o t d e f i n i t i v e l y e x c l u d e t h i s mechanism b e c a u s e we do n o t know the n o r m a l modes o f a condensed i s o p r o p y l benzene. A more l i k e l y mechanism i s t h e

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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135

c o u p l i n g o f t h e m o l e c u l a r i o n s t a t e t o d i p o l e a c t i v e n o r m a l modes o f n e i g h b o r i n g m o l e c u l e s v i a l o n g - r a n g e Coulomb i n t e r a c t i o n s . The c o n s e q u e n c e s of this i n t e r a c t i o n c a n be d e s c r i b e d by a g e n e r a l i z a t i o n o f the H u a n g - R h y s m o d e l ( 4 4 , 4 5 , 4 6 ) , t h a t was p r o p o s e d t o i n t e r p r e t t h e e a r l i e r XPS s t u d i e s o f a l k a l i h a l i d e s (25). A r e c e n t mode^l c a l c u l a t i o n has been c a r r i e d o u t ( 2 1 ) t h a t p r e d i c t s c = 4 x 10 eV(°K) f o r c o n d e n s e d b e n z e n e , a v a l u e in g o o d ^ o r r e g p o n d e n c e w i t h t h e o b s e r v e d v a l u e o f c = 2 . 6 + 0 . 5 x 10 eV ( K) f o r i s o p r o p y l benzene o b t a i n e d from F i g . 4 . From t h i s c l o s e c o r r e s p o n d e n c e we have i n f e r r e d t h a t c o u p l i n g o f a g i v e n i s o p r o p y l benzene m o l e c u l a r i o n s t a t e to i n f r a - r e d a c t i v e v i b r a t i o n s o f n e i g h b o r i n g n e u t r a l i s o p r o p y l benzene m o l e c u l e s i s the p r e d o m i n a n t c a u s e o f t h e o b s e r v e d t e m p e r a t u r e dependent l i n e width. The o r i g i n of the temperature-independent solid state b r o a d e n i n g , 0 . 4 eV A < 0 . 6 eV we a t t r i b u t e d t o s p a t i a l v a r i a t i o n s i n the e l e c t r o n i r e l a x a t i o n e n e r g i e s i n th S i m i l a r w i d t h s ( t o 0 . 6 eV) have been o b s e r v e d i n a v a r i e t y o f other contexts, i n c l u d i n g condensed t h i n f i l m s o f and CO m o l e c u l e s ( 2 8 ) and t h e s u b - m o n o l a y e r a d s o r p t i o n o f t h e s e m o l e c u l e s on m e t a l s u r f a c e s (29). I n t e r a t o m i c A u g e r and e l e c t r o n - h o l e shakeup p r o c e s s e s have b e e n p r o p o s e d , b u t found t o be t o o s m a l l t o account f o r the observed w i d t h s i n these cases ( 2 8 , 4 7 , 4 8 ) . On an e n e r g e t i c b a s i s t h e s e l a t t e r p r o p o s e d p r o c e s s e s a r e e x p e c t e d t o o c c u r f o r i n n e r v a l e n c e o r b i t a l s and n o t f o r t h e o u t e r - m o s t I P studied here. A l s o , t h e y c o n t a i n no s i g n i f i c a n t t e m p e r a t u r e dependence. By t h e p r o c e s s o f e l i m i n a t i o n , s p a t i a l v a r i a t i o n s i n the r e l a x a t i o n e n e r g i e s are the o n l y extent i n t e r p r e t a t i o n o f the temperature-independent (inhomogeneous) b r o a d e n i n g . The s t u d y o f i s o p r o p y l benzene c a n be summarized as f o l l o w s . The w i d t h o f t h e UPS l i n e c o r r e s p o n d i n g t o r e m o v a l o f t h e l o w e s t b i n d i n g energy i r - e l e c t r o n i s temperature dependent. T h i s temp e r a t u r e dependence c o n t r i b u t e s s i g n i f i c a n t l y t o t h e UPS l i n e w i d t h at e l e v a t e d t e m p e r a t u r e s . The f a c t t h a t the w i d t h i s temperature dependent indicates that the mechanism i n v o l v e s vibrations. A l t h o u g h e x p e r i m e n t a l l y , i n t r a m o l e c u l a r and i n t e r molecular effects c o u l d n o t be s e p a r a t e d , t h e o r e t i c a l models p r e d i c t t h a t o f the e f f e c t i s m o s t l y i n t e r m o l e c u l a r . The s m a l l r e s i d u a l l i n e w i d t h o b s e r v e d i s due t o sample i n h o m o g e n e i t i e s . P r e s u m a b l y , an i d e a l s i n g l e c r y s t a l t h i n f i l m w o u l d e x h i b i t t h e same A(T) b u t have a s m a l l e r A . R e l a x a t i o n E n e r g y S h i f t s and L o c a l i z e d M o l e c u l a r - I o n S t a t e s i n A r o m a t i c P e n d a n t Group P o l y m e r s . The e l e c t r o n i c s t r u c t u r e o f p o l y s t y r e n e ( P S ) and p o l y v i n y l p y r i d i n e ( P V P ) have b e e n s t u d i e d u s i n g a v a r i e t y o f e l e c t r o n i c s p e c t r o s c o p i e s and m o d e l c a l c u l a tions (20). H e r e , we r e v i e w the r e s u l t s o f t h e UPS and u l t r a v i o l e t a b s o r p t i o n s p e c t r o s c o p y (UAS) p o r t i o n o f t h a t s t u d y , and d i s c u s s t h e r e s u l t s i n a phenomeno l o g i c a l manner. The a i m o f t h i s

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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s t u d y was to c h a r a c t e r i z e the e l e c t r o n i c e x c i t a t i o n s o f the p o l y m e r s i n terms o f t h o s e o f t h e m o l e c u l a r b u i l d i n g b l o c k s o f t h e polymers. To t h i s e n d , t h e a p p r o p r i a t e model monomer m o l e c u l e s were s t u d i e d i n b o t h the gas phase and the c o n d e n s e d m o l e c u l a r s o l i d phases. Some t e c h n i c a l d e t a i l s o f the p h o t o e l e c t r o n s p e c t r o s c o p y o f t h e p o l y m e r f i l m s a r e g i v e n i n the A p p e n d i x . Some o f the g e n e r a l p r a c t i c e s d i s c u s s e d t h e r e a p p l y t o t h e condensed m o l e c u l a r s o l i d s as w e l l . A t t h i s p o i n t i t i s w o r t h r e p h r a s i n g some o f t h e i s s u e s o f the above d i s c u s s i o n s . The UPS s p e c t r a a r e a measure o f t h e s i n g l e - p a r t i c l e e x c i t a t i o n s p e c t r u m o f the m o l e c u l e , i n so f a r as r e m o v a l o f an e l e c t r o n i s c o n c e r n e d , w h i l e UAS d a t a a r e a measure o f the p a r t i c l e - h o l e e x c i t a t i o n s p e c t r u m . I n o t h e r t e r m s , UPS m e a s u r e s the m o l e c u l a r - i o n s t a t e s w h i l e UAS m e a s u r e s e x c i t e d s t a t e s o f the n e u t r a l m o l e c u l e . F o r a m o l e c u l e i n i s o l a t i o n , i n a o n e - e l e c t r o n p i c t u r e the v a l e n c e e l e c t r o n m o l e c u l a r c a t i o n s t a t e s are c o m p r i s e d o f the s e ( M O s ) c o n t a i n i n g one h a l f - f i l l e u l a r o r b i t a l and t h e t o t a l i t y o f o t h e r f u l l y o c c u p i e d o r b i t a l s , d i s t o r t e d from t h e i r s i t u a t i o n i n the n e u t r a l m o l e c u l e due t o t h e r e m o v a l o f an e l e c t r o n from the m o l e c u l e i n a p h o t o e l e c t r o n t r a n s i t i o n (21_). The c h a r g e d e n s i t y change t h a t o c c u r s upon t h e g e n e r a t i o n o f a h o l e - s t a t e r e s u l t s i n a r e o r g a n i z a t i o n o f the t o t a l charge d i s t r i b u t i o n o f the m o l e c u l e . The differences b e t w e e n the m o l e c u l a r c a t i o n s t a t e s measured by p h o t o e l e c t r o n s p e c t r o s c o p y and the e i g e n s t a t e s o f t h e n e u t r a l m o l e c u l e a r e t h e n j u s t t h o s e d i c t a t e d by t h e r e d i s t r i b u t i o n o f t o t a l c h a r g e ( r e o r g a n i z a t i o n ) as w e l l as t h e a c c o m p a n y i n g change i n e l e c t r o n c o r r e l a t i o n (1J5, 16^) ( n e g l e c t i n g r e l a t i v i s t i c e f f e c t s ) . The e n e r g y g a i n e d by the m o l e c u l e i n t h e e x c i t e d s t a t e i s termed t h e i n t r a m o l e c u l a r r e l a x a t i o n e n e r g y , o f w h i c h t h e r e a r e a t o m i c (a5 and e l e c t r o n i c ( e ) c o n t r i b u t i o n s (21). T h i s energy i s sometimes c a l l e d the Koopman's d e f e c t , since i t is this energy which a c c o u n t s f o r t h e d i s c r e p e n c y between Koopman's t h e o r e m and o b s e r v e d UPS s p e c t r a i n a o n e - e l e c t r o n f o r m a l i s m (15^, 1 6 ) . f

I n a condensed m o l e c u l a r s o l i d , however, t h e r e are also intermolecular r e l a x a t i o n ( p o l a r i z a t i o n ) effects that occur i n a d d i t i o n to the i n t r a m o l e c u l a r e f f e c t s (jLO, 11), as d i s c u s s e d above. I n f a c t , i n any d i e l e c t r i c medium, the t o t a l n e t p o s i t i v e c h a r g e d e n s i t y on the m o l e c u l a r c a t i o n i n d u c e s corresponding e l e c t r o n i c , and u l t i m a t e l y a t o m i c , d i s t o r t i o n s i n t h e s u r r o u n d i n g med i u m . The c l e a r e s t e v i d e n c e f o r t h e v a l i d i t y o f t h e m o l e c u l a r - i o n c o n c e p t i s t h e o b v i o u s s i m i l a r i t y b e t w e e n t h e UPS and UAS s p e c t r a o f PVP and t h o s e o f i t s c o r r e s p o n d i n g m o d e l m o l e c u l e 2 - e t h y l p y r i d i n e . These s p e c t r a a r e shown i n F i g s . 5 and 7 , r e s p e c t i v e l y . In p a r t i c u l a r , i n F i g . 5 t h e UPS s p e c t r a f o r t h e s e r i e s o f materials pyridine(v), 2-ethyl pyridine(v), 2-ethyl pyridine(s), and PVP a r e p r e s e n t e d i n o r d e r t o show e x p l i c i t l y t h e r e l a t i v e c o n s e q u e n c e s o f t h e 2 - e t h y l s u b s t i t u e n t and c o n d e n s a t i o n effects

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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UPS(s) 40.8 eV (m) PHOTONS "H^ UPS(s), 40.8 eV (m) PHOTONS

.

.

N.

UPS(v) 408eV-5 PHOTONS

fi^l N^J

PYRIDINE(v), 21.2 eV -2^/1 M ^ l Al PHOTONS Aj [/ I /V (TURNER, ET AL) ^ ^ 25

_l_ 20 15 10 5 BINDING ENERGY (eV) FOR POLYMER CURVE

Figure 5. UV photoemission spectroscopy data for the series of materials pyridine (v) (2), 2-ethyl pyridine (s), and poly(2-vinyl pyridine) (s). The shift of 0.5 eV between the lowest binding-energy peak for pyridine and for 2-ethy pyridine reflects the effects of hyperconjugation induced by the ethyl group substituen The additional shift of 1.5 eV between the vapor (v) and solid-state (s) data on 2-eth pyridine arises from the intermolecular relaxation effects discussed in text. The spectr are presented in the figure shifted relative to each other by an amount such that t lowest energy ionization potentials are aligned. The absolute values of these ionizatio potentials are indicated in the figure.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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+-

H C

H

UPS(s),408eV (m) -w PHOTONS

c

- 2 i-

n

POLYSTYRENE | UPS(s), 408 eV(m) PHOTONS-" \ UPS(v), 40.8eV PHOTONS" ETHYL BENZENE

UPS(v), 21.2 eV PHOTONS

;

3

UPS(v), 21.2 PHOTONS^?, (TURNER, ET AL)

Figure 6. UV photoemission spectroscopy data for the series of materials benzene (v) (3), 2-ethyl benzene (v), 2-ethyl benzene (s), and polystyrene (s). The shift of 0.14 eV between the lowest binding-energy peak for benzene and the 2-ethyl benzene reflects the effects of hyperconjugation induced by the ethyl group substituent. The additional shift of 1.5 eV between the vapor (v) and solid-state (s) spectra on 2-ethyl benzene arises from the intermolecular relaxation effects discussed in the text. The spectra are presented in thefigureshifted relative to each other by an amount such that the lowest energy ionization potentials are aligned. The absolute values of these ionzation potentials are indicated in the figure.

Figure 7. UV absorption spectra in the region of the first absorption band in pyridine CA -» B B ) for 2-ethyl pyridine and poly(2-vinyl pyridine). The similarity of these two spectra indicates that the aromatic pyridine group determines these lowest energy optical excitations of the polymer. The slight relaxation energy shi is discussed in the text. The vibrational progression involving the rj' vibration of pyridine (50, 62) is clearly visible. 1

t

1

lt

2

10

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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i n t o a s o l i d on t h e e v a l u a t i o n o f t h e s p e c t r u m o f PVP f r o m t h a t o f i t s p y r i d i n e pendant-group moiety. I n t h e above n o t a t i o n , v r e f e r s to the vapor phase, w h i l e s r e f e r s to s o l i d phase. Based upon t h e s e s p e c t r a i n c o n j u n c t i o n w i t h model MO c a l c u l a t i o n s ( 2 0 ) , i t i s c l e a r t h a t t h e uppermost t h r e e i o n i z a t i o n p o t e n t i a l s i n P V P , e m a n a t i n g f r o m 2b^(7r), laA *), and l l a ^ ( a ) s t a t e s o f p y r i d i n e a r e p r e s e r v e d t h r o u g h o u t t h i s sequence o f m a t e r i a l s , a l t h o u g h the r e l a t i v e o r d e r i n g o f t h e s e s t a t e s i s a l t e r e d by t h e 2-ethyl substitution (20). S i m i l a r l y , the d e t a i l e d resemblance b e t w e e n the l o w e s t e n e r g y (TT •> TT*) U V a b s o r p t i o n (UAS) b a n d s i n PVP and 2 - e t h y l p y r i d i n e ( 5 0 ) shown i n F i g . 7 , d e m o n s t r a t e s t h a t the l o w - e n e r g y e l e c t r o n i c t r a n s i t i o n s i n n e u t r a l PVP a r e e s s e n t i a l l y t h o s e o f 2 - e t h y l p y r i d i n e . C o n s e q u e n t l y , we see t h a t t h e l o w - e n e r g y e l e c t r o n i c e x c i t a t i o n s o f PVP a r e essentially t h o s e o f the p y r i d y l e t h y l e n e r e p e a t u n i t s w h i c h make up P V P . W i t h i n the c o n t e x t o f the uv a b s o r p t i o n bands o f substituted polyethylenes this resul p o l y s t y r e n e (PS (51^, 5 2 ) observations to i n c l u d e m o l e c u l a r c a t i o n s t a t e s ( i . e . , photoe m i s s i o n s p e c t r a ) i n a d d i t i o n to m o l e c u l a r e x c i t o n s t a t e s . The UPS and UAS s p e c t r a f o r PS a n a l o g o u s t o t h o s e shown i n F i g s . 5 and 7 f o r PVP a r e g i v e n i n F i g ' s 6 and 8 f o r t h e s e r i e s b e n z e n e ( v ) , ethyl benzene(v), ethyl benzene(s), and P S . I t i s c l e a r from F i g ' s . 6 and 8 and t h e l i t e r a t u r e on the uv a b s o r p t i o n p r o p e r t i e s o f s u b s t i t u t e d p o l y e t h y l e n e s ( 5 1 , 5 2 , 53) t h a t the l o w - e n e r g y e l e c t r o n i c e x c i t a t i o n s o f these m a t e r i a l s are p r o p e r l y i d e n t i f i e d as b e i n g l o c a l i z e d on t h e p e n d a n t - g r o u p m o i e t i e s . We c o n c l u d e , t h e r e f o r e , t h a t t h e m o l e c u l a r - i o n c o n c e p t i s a d e q u a t e l y b a s e d on s p e c t r o s c o p i c d a t a i n the case o f a r o m a t i c s u b s t i t u t e d p o l y e t h y l enes . 1

F i g u r e s 5-8 show f u r t h e r t h a t the two f e a t u r e s o f the c o n d e n s e d phase s p e c t r a w h i c h d i f f e r from t h e i r g a s - p h a s e c o u n t e r p a r t s a r e the e n e r g i e s and w i d t h s o f the i n d i v i d u a l i o n i z a t i o n p e a k s or o p t i c a l - a b s o r p t i o n l i n e s . I n b o t h t h e UPS and UAS s p e c t r a t h e l i n e s i n the s o l i d s t a t e a r e s h i f t e d t o l o w e r e n e r g i e s r e l a t i v e to t h e c o r r e s p o n d i n g ones i n t h e gas phase by a r e l a x a t i o n energy, ^ 1 eV f o r i o n s t a t e s and ^ 0 . 1 eV f o r e x c i t o n s t a t e s , a s s o c i a t e d w i t h the i n t e r m o l e c u l a r p o l a r i z a t i o n i n d u c e d by the i o n o r e x c i t o n ( 1 0 , _19, 2 0 , 2 1 , 2 2 ) . I n a d d i t i o n , these l i n e s a l s o a r e b r o a d e r i n t h e s o l i d s t a t e , due i n p a r t t o t h e i n t e r a c t i o n o f the i o n s and e x c i t o n s w i t h i n t e r m o l e c u l a r p o l a r i z a t i o n f l u c t u a t i o n s (_19, 2 0 , 2 1 , 2 2 , 3 6 , 54) and t o a l e s s e r e x t e n t to t h e s p a t i a l i n h o m o g e n e i t y o f t h e p o l a r i z a t i o n - i n d u c e d r e l a x a t i o n e n e r g y i n the v i c i n i t y o f a s u r f a c e (_18, _19, 3 7 ) . It i s i m p o r t a n t t o n o t e i n F i g ' s 5 and 6 , h o w e v e r , t h a t f o r h o l e s t a t e s i n b o t h PVP and P S , t h e s e s o l i d - s t a t e e f f e c t s a r e e s s e n t i a l l y f u l l y m a n i f e s t e d i n the condensed m o l e c u l a r s o l i d f i l m s . I n f a c t , b o t h t h e i n t e r m o l e c u l a r r e l a x a t i o n e n e r g i e s , E = 1.5 ± 0 . 1 e V , and s o l i d - s t a t e - r r - e l e c t r o n l i n e w i d t h s , Air ^ 1.0 e V , = a r e the same w i t h i n e x p e r i m e n t a l e r r o r f o r the p o l y m e r and the condensed m o l e c u l a r s o l i d f i l m s .

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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The r e s u l t s d e r i v e d from F i g ' s . 5 and 6 a r e t h a t the i n t e r m o l e c u l a r r e l a x a t i o n ( p o l a r i z a t i o n energy) s h i f t s are e s s e n t i a l l y u n i f o r m f o r a l l the m o l e c u l a r c a t i o n s t a t e s i . e . , E % 1.5 eV i n d e p e n d e n t o f the i d e n t i f y o f the MO i n v o l v e d , t h a t t h e v a l u e s o f these s h i f t s a r e i d e n t i c a l f o r PVP and P S , and t h a t the l i n e w i d t h s o f the uppermost i r - e l e c t r o n c a t i o n s t a t e s a r e i d e n t i c a l i n PVP and P S . The e q u a l i t y o f the i n t e r m o l e c u l a r r e l a x a t i o n e n e r g i e s f o r PS and PVP i s s u r p r i s i n g b e c a u s e o f the e x i s t a n c e o f a much l a r g e r v a l u e o f the s t a t i c d i e l e c t r i c c o n s t a n t i n ( p o l a r ) PVP t h a n i n ( n o n p o l a r ) P S , l e a d i n g to the p r e d i c t i o n o f a c o r r e s p o n d i n g l y l a r g e r r e l a x a t i o n e n e r g y i n the B o r n t h e o r y o f s o l v a t i o n , w h i c h had been commonly used to model p o i n t c h a r g e s i n d i e l e c t r i c m e d i a (2X)). M o r e o v e r , t h i s d i s c r e p a n c y t r a n s c e n d s t h e a d i a b a t i c as w e l l as the i n d e p e n d e n t - e l e c t r o n v e r s i o n s o f the theory of d i e l e c t r i c r e l a x a t i o n (55). T h e r e f o r e our e x p e r i m e n t a l UPS d a t a shown i n F i g s . 5 and 6 have r e v e a l e d a f u n d a m e n t a l f a i l u r e of previous theorie t i o n w h i c h have been show m o l e c u l a r - i o n model ( 2 0 ) . S p e c i f i c a l l y , i n the B o r n t h e o r y m o d e l o f s o l v a t i o n , t h e i n t e r m o l e c u l a r r e l a x a t i o n energy i s (21) E

=

r

f R

[l- (0) e

1

(6)

],

where e i s the e l e c t r o n i c c h a r g e , R i s the r a d i u s o f a s p h e r e o f v o l u m e e q u i v a l e n t t o t h a t o f the m o l e c u l e i n q u e s t i o n , and e ( 0 ) i s the z e r o f r e q u e n c y ( s t a t i c ) d i e l e c t r i c c o n s t a n t . Since there is o v e r a f a c t o r o f two d i f f e r e n c e between e ( 0 ) f o r PVP and P S , t h e B o r n model p r e d i c t s a E o f a b o u t 4 . 5 eV f o r PVP and a b o u t 1.3 eV f o r P S ; i n d i s a g r e e m e n t w i t h the o b s e r v e d r e s u l t o f 1.5 ± 0 . 1 eV f o r b o t h PS and P V P . A m o l e c u l a r i o n model t a k i n g i n t o a c c o u n t t h e i n t e r a c t i o n s o f the e l e c t r o n i c ( i o n i c ) s t a t e s w i t h the l o n g i t u d i n a l p o l a r i z a t i o n f l u c t u a t i o n s o f the ( d i e l e c t r i c ) medium, h o w e v e r , a c c o u n t s f o r the o b s e r v a t i o n s ( 2 0 ) . The low f r e q u e n c y t o r s i o n a l and i . r . l o n g i t u d i n a l p o l a r i z a t i o n modes a r e s t r o n g l y c o u p l e d to the e l e c t r o n i c e x c i t a t i o n s . Therefore these f l u c t u a t i o n s h o m o g e n e o u s l y b r o a d e n the UPS l i n e s r a t h e r t h a n s h i f t them ( 2 J J . The w e a k l y c o u p l e d b r a n c h e s o f the d i e l e c t r i c f u n c t i o n t h e n c o n t r i b u t e to the i n t e r m o l e c u l a r r e l a x a t i o n i n the form ( 5 6 ) , (7) where e (u)) i s the v a l u e o f e( w) f o r j u s t above the e l e c t r o n i c e x c i t a t i o n r e g i o n o f the e(u)) curve, while ^(w) i s the v a l u e j u s t b e l o w the e l e c t r o n i c e x c i t a t i o n r e g i o n o f ( ) . S i n c e the e l e c t r o n i c c o n t r i b u t i o n s to t h e d i e l e c t r i c c o n s t a n t , ( ) , for 1 eV < Ha) < 10 e V , a r e e s s e n t i a l l y i d e n t i c a l f o r PS and P V P , t h e s e w e a k l y c o u p l e d modes a r e e x p e c t e d to p r o d u c e the same r e s u l t i n a

e

w

e

w

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

11.

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t h e i n t e r m o l e c u l a r r e l a x a t i o n s h i f t i n t h e UPS s p e c t r a f o r b o t h PS and P V P ; as o b s e r v e d . I n a d d i t i o n , the a s s o c i a t e d homogeneous c o n t r i b u t i o n s to the l i n e w i d t h s are A ^ 0.3 eV. S i n c e t h i s v a l u e i s much s m a l l e r t h a n t h e p r e d i c t e d inhomogeneous s t a t i c - f l u c t u a t i o n - i n d u c e d v a l u e o f a b o u t 0 . 7 5 eV ( 2 1 ) , we e x p e c t t h e o b s e r v e d UPS l i n e s t o be i n h o m o g e n e o u s l y b r o a d e n e d and a b o u t t h e same w i d t h i n b o t h PS and P V P , a g a i n i n a c c o r d a n c e w i t h t h e m e a s u r e m e n t s . The q u a n t i t a t i v e p r e d i c t i o n o f t h e e x p e r i m e n t a l r e s u l t s by t h e m o l e c u l a r i o n m o d e l c o n c e p t a l s o i s v e r y good ( 2 0 ) . The f i n a l p o i n t t o be d i s c u s s e d i n v o l v e s t h e g a s - t o - s o l i d s h i f t s i n the o p t i c a l a b s o r p t i o n s p e c t r a . Having e s t a b l i s h e d the correspondence between the e l e c t r o n i c e x c i t a t i o n s o f the c o n d e n s e d monomer f i l m s and t h e p o l y m e r f i l m s , we c a n f o c u s on a c o m p a r i s o n o f t h e gas phase monomer UAS s p e c t r a w i t h t h e p o l y m e r film spectra. A s shown i n F i g ' s 7 and 8 t h e p o l y m e r UAS s p e c t r a a r e s h i f t e d and b r o a d e n e d r e l a t i v e t o t h e i r gas phase c o u n t e r parts. As i n t h e c a s e a g a i n be q u a n t i t a t i v e l y u l a r i o n framework. The UAS i n t e r m o l e c u l a r r e l a x a t i o n e n e r g y s h i f t i s t w i c e as l a r g e i n PVP ( 0 . 0 9 eV) as i n PS ( 0 . 0 4 e V ) . T h i s o b s e r v a t i o n i s c o n s i s t e n t w i t h t h e e x i s t a n c e o f a l a r g e r number o f w e a k l y c o u p l e d l o n g i t u d i n a l p o l a r i z a t i o n modes i n PVP as compared with PS. I n a d d i t i o n , t h e b r o a d e n i n g o f the gas phase 2 - e t h y l p y r i d i n e UAS s p e c t r a as compared w i t h t h e gas p h a s e e t h y l b e n z e n e s p e c t r a i s an i n d i c a t i o n t h a t t h e r e a r e more s t r o n g l y c o u p l e d i n t r a m o l e c u l a r modes i n 2 - e t h y l p y r i d i n e t h a n i n e t h y l b e n z e n e ; a f a c t c o n s i s t e n t w i t h the p o l a r n a t u r e o f 2 - e t h y l p y r i d i n e . F i n a l ly, t h e s o l i d s t a t e ( p o l y m e r ) UAS s p e c t r a f o r PVP a r e more s e v e r e l y broadened t h a n t h e UAS s p e c t r a f o r P S ; once again c o n s i s t a n t w i t h the e x i s t e n c e the l a r g e r number o f i . r . a c t i v e modes i n PVP ( 5 6 ) . h

The p u r p o s e o f t h i s s e c t i o n has b e e n t o u t i l i z e a m o l e c u l a r i o n c o n c e p t t o p r o v i d e a s a t i s f a c t o r y i n t e r p r e t a t i o n o f t h e UPS and UVA d a t a g i v e n i n F i g s . 5 , 6 , 7 and 8 . I n p a r t i c u l a r , the m o l e c u l a r i o n c o n c e p t a f f o r d s an e l e m e n t a r y p r e d i c t i o n o f t h e s u r p r i s i n g r e s u l t t h a t the measured i n t e r m o l e c u l a r r e l a x a t i o n e n e r g i e s a r e t h e same f o r b o t h PVP and PS ( 2 0 ) even t h o u g h t h e f o r m e r i s a p o l a r m a t e r i a l and w h i l e t h e l a t t e r i s n o t ( i . e . , t h e two m a t e r i a l s e x h i b i t q u i t e d i f f e r e n t l o w - f r e q u e n c y d i e l e c t r i c responses). We summarize h e r e t h e f e a t u r e s o f t h e UPS and UVA d a t a w h i c h l e a d t o the m o l e c u l a r i o n c o n c e p t f o r t h e s e a r o m a t i c p e n d a n t g r o u p polymers. F i r s t , t h e s p e c t r a o f the p o l y m e r s PVP and PS a r e e s s e n t i a l l y i d e n t i c a l to t h o s e o f c o n d e n s e d model m o l e c u l a r moieties 2-vinyl pyridine and e t h y l benzene, respectively. S e c o n d , the s o l i d - s t a t e s p e c t r a a r e r e l a t e d to t h e g a s - p h a s e s p e c t r a o f t h e s e model m o i e t i e s by an e s s e n t i a l l y c o n s t a n t s h i f t to h i g h e r e n e r g y ( l o w e r b i n d i n g e n e r g y ) o f a l l t h e i o n i z a t i o n p e a k s by E = 1.5 ± 0 . 1 e V . T h i r d , the w i d t h i n energy o f the s o l i d - s t a t e i o n i z a t i o n peak i s ATT = 1.0 0.1 f o r b o t h polymers r

±

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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to

o Figure 8. UV absorption spectra in the region of the first absorption band in benzene (1A -> 1B ) f° the zene in the gas phase and The similarity of the molecular and polymer spectra indicates that the aromatic groups determine these lowest energy optical excitations in the polymer. The slight relaxation energy shift is discussed in the text. The vibrational progressions of both ethyl benzene and PS are clearly seen. r

19

Figure 9.

2u

—H-i

1—H

1



1

i

2200 2400 2600 2800 WAVELENGTH (ANGSTROMS)

Gas-phase UPS spectra of ethylene molecules (2) compared with that of polyacetylene (51)

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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a t T = 3 0 0 ° K , a l t h o u g h i t s t e m p e r a t u r e dependence was n o t measured. The t e m p e r a t u r e e f f e c t was i n v e s t i g a t e d , h o w e v e r , e m p l o y i n g m o d e l - m o l e c u l e s t u d i e s , i . e . i s o p r o p y l b e n z e n e as d i s c u s s e d above. The s i m i l a r i t y o f t h e p o l y m e r UPS d a t a and t h o s e o f t h e g a s phase model m o l e c u l a r m o i e t i e s i s i n c o r p o r a t e d i n t o a m o l e c u l a r i o n framework by e n v i s a g i n g the i o n s t a t e s i n t h e s o l i d t o be t h o s e o f the i s o l a t e d model m o i t i e s m o d i f i e d o n l y by their i n t e r a c t i o n w i t h p o l a r i z a t i o n f l u c t u a t i o n s i n the surrounding d i e l e c t r i c medium. I f t h e s e f l u c t u a t i o n s c a u s e l a r g e enough v a r i a t i o n s i n the r e l a x a t i o n e n e r g i e s , they l o c a l i z e the i o n s t a t e s i n the polymer onto i n d i v i d u a l pendant-group moieties. T h a t such i s t h e c a s e i n PVP and PS was d i s c u s s e d a b o v e . This f l u c t u a t i o n - i n d u c e d l o c a l i z a t i o n s u f f i c e s to e x p l a i n the d e t a i l e d s i m i l a r i t y b e t w e e n t h e UPS d a t a o f t h e c o n d e n s e d model m o l e c u l e s and t h o s e o f t h e c o r r e s p o n d i n g p o l y m e r ( F i g s 5 and 7) f o r t h o s e low-ionization-potentia t r i b u t i o n s from t h e v a l e n c alkane backbone. I n a d d i t i o n , t h e s h i f t s and b r o a d e n i n g o f g a s phase s p e c t r a l l i n e s i n t h e s o l i d s t a t e a r e q u a n t i t a t i v e l y p r e d i c t e d by a m o l e c u l a r i o n m o d e l ( 2 0 , 21_ 5 6 ) , t h e l i t e r a t u r e f o r w h i c h s h o u l d be r e f e r r e d t o f o r a d e t a i l e d m a t h e m a t i c a l d i s c u s s ion. 9

E l e c t r o n i c States of Polyacetylene. In contrast to the l o c a l i z e d m o l e c u l a r i o n s t a t e s seen i n t h e s p e c t r a o f PS and PVP i n t h e p r e v i o u s s e c t i o n , we o u t l i n e h e r e v e r y b r i e f l y t h e r e s u l t s o f a s t u d y o f the f u l l y c o n j u g a t e d p o l y m e r , p o l y a c e t y l e n e , o r ( C H ) , u s i n g p h o t o e l e c t r o n s p e c t r o s c o p y (57^, 5 9 ) . T h i s work was s t i m u l a t e d by t h e enormous change i n e l e c t r i c a l c o n d u c t i v i t y o f f r e e s t a n d i n g f i l m s o f (CH) upon d o p i n g ( 6 0 ) . The e l e c t r o n i c s t r u c t u r e o f p o l y a c e t y l e n e has b e e n a s u b j e c t o f c o n t r o v e r s y f o r decades ( 5 7 ) . F o r t h e p r e s e n t p u r p o s e s , we f o c u s on what w o u l d be the d e l o c a l i z e d n a t u r e o f the l o w e s t b i n d i n g energy ir-band (57) i n an i s o l a t e d i d e a l c h a i n o f (CH) and compare i t w i t h t h e l b ^ ( TT ) state i n ethylene. A l t h o u g h a c e t y l e n e has a t r i p l e C=C Bond, p o l y a c e t y l e n e c o n t a i n s a l t e r n a t i n g d o u b l e C=C and s i n g l e C-C b o n d s . The e t h y l e n e m o l e c u l e , t h e r e f o r e , i s a good model m o l e c u l e r e p r e s e n t a t i v e o f the l o c a l e l e c t r o n i c s t r u c t u r e i n (CH) . The lowest b i n d i n g energy i r - s t a t e , however, i s d e l o c a l i z e d over the e n t i r e (CH) c h a i n . R e s i d u a l e l e c t r i c a l c o n d u c t i v i t y a r i s e s from b o n d - a l t e r n a t i o n - d e f e c t s f r o z e n i n t o the polymer f i l m d u r i n g the f i l m growth, using a Z i e g l e r - N a t t a c a t y l i s t (60). More r e c e n t models (62) o f the e l e c t r i c a l c o n d u c t i v i t y i n v o k e q u a s i - l o c a l i z e d states i n a Fermi glass d e s c r i p t i o n . x

The m a g n i t u d e o f t h e i n t e r m o l e c u l a r r e l a x a t i o n e n e r g y s h i f t s i n t h e s o l i d phase UPS s p e c t r a o f t h e p o l y m e r f i l m s r e l a t i v e t o t h e gas phase UPS s p e c t r a o f t h e m o d e l m o l e c u l e s s e e n f o r t h e a r o m a t i c p e n d a n t group p o l y m e r s i s a l s o o b s e r v e d i n t h e c a s e o f t h e (CH) v e r s u s e t h y l e n e UPS s p e c t r a , as c a n be seen i n F i g . 9 .

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The f a c i l i t y f o r u n d e r s t a n d i n g the i n t e r m o l e c u l a r r e l a x a t i o n e n e r g y s h i f t i s c o n t a i n e d w i t h i n the framework o f t h e m o l e c u l a r i o n c o n c e p t and i s c o n s i s t e n t w i t h r e c e n t m o d e l s ( 5 6 , 62) o f t h e v e r y ir-band edges i n (CH) . S i n c e t h e o p t i c a l a b s o r p t i o n i n ( C H ) has b e e n shown t o be e x c i t i o n i c i n n a t u r e (_57), w i t h l a r g e m o l e c u l a r e x c i t o n b i n d i n g e n e r g i e s (^ 1 e V ) , t h e v e r y band edge s t a t e s a r e s a i d t o be c o n s i s t e n t w i t h a F e r m i g l a s s m o d e l . Thus t h e localization a l l o w s an i n t e r p r e t a t i o n o f the intermolecular r e l a x a t i o n e n e r g y s h i f t on t h e same g r o u n d s as f o r t h e a r o m a t i c p e n d a n t group p o l y m e r s . R e c e n t p h o t o v o l t a i c e f f e c t measurements ( 6 3 ) have v e r i f i e d t h e e x i s t a n c e o f an e x c i t o n i c a b s o r p t i o n edge a t a p h o t o n e n e r g y c o r r e s p o n d i n g t o what i s c a l l e d t h e gap e n e r g y i n (CH) and the e x i s t a n c e o f a f r e e e - h p a i r a b s o r p t i o n edge a b o u t 1*1/2 eV t o h i g h e r p h o t o n e n e r g y . These o b s e r v a t i o n s t h u s v i n d i c a t e the o b s e r v a t i o n o f t h e i n t e r m o l e c u l a r r e l a x a t i o n e n e r g y i n t h e UPS s p e c t r a t h a t otherwis expected onl fo molecula insulating solids with localize x

Appendix: Sample P r e p a r a t i o n and some o t h e r E x p e r i m e n t a l D e t a i l s R e l e v a n t t o t h e S t u d y o f P o l y s t y r e n e and P o l y ( 2 - V i n y l P y r i d i n e ) A 4 0 . 8 eV UPS s p e c t r u m o f a n o m i n a l l y 2oX t h i c k f i l m o f p o l y s t y r e n e ( P S ) i s a l s o shown i n F i g . 6 . R a t h e r c a r e f u l s o l u t i o n c a s t i n g p r o c e d u r e s , coupled w i t h immediate i n s e r t i o n i n t o the s p e c t r o m e t e r vacuum chamber f o r d r y i n g , was n e c e s s a r y t o p r o d u c e f i l m s o f u n i f o r m t h i c k n e s s , r e l a t i v e l y f r e e from s u r f a c e c o n t a m ination. The r a t i o n a l e f o r t h e t h i n samples was d i s c u s s e d a b o v e . The 4 0 . 8 eV He I I r e s o n a n c e l i n e was c h o s e n f o r use f o r two r e a s o n s : i t s e n e r g y , when u s e d w i t h a monochromator t o f i l t e r o u t o t h e r l i n e s , e n a b l e s e x a m i n a t i o n o f a w i d e r range o f energy o f MO's t h a n t h e u s u a l 21.2 eV He I r e s o n a n c e l i n e ; and t h e r e i s l e s s influence o v e r most o f t h e s p e c t r u m from b a c k g r o u n d due t o inelastic electrons. The e f f e c t o f s u r f a c e c o n t a m i n a t i o n was s t u d i e d by c o m p a r i n g 21.2 eV and 4 0 . 8 eV UPS s p e c t r a , as w e l l as by s u b s e q u e n t l y e x p o s i n g t h e s a m p l e s t o l a b o r a t o r y a i r f o r s h o r t p e r i o d s o f t i m e and t h e n e x a m i n i n g the s p e c t r a . The s l i g h t peak a t a b o u t 14 eV b i n d i n g e n e r g y i n t h e PS s p e c t r u m r e l a t i v e t o t h e s p e c t r u m o f c o n d e n s e d e t h y l b e n z e n e , i n F i g . 6 , i s due t o s u r f a c e contamination. The f i r s t o p t i c a l a b s o r p t i o n bands o f b e n z e n e and p y r i d i n e a r e known t o o c c u r n e a r 5 e V . The o p t i c a l d a t a on 1000 A t h i c k , s o l u t i o n c a s t , PS and PVP as w e l l as on t h e m o d e l m o l e c u l e s e t h y l b e n z e n e and 2 - e t h y l p y r i d i n e i n the gas phase were r e c o r d e d on a C a r y 14 d o u b l e beam i n s t r u m e n t . The d a t a a r e t h e r e f o r e l i m i t e d t o K w < 6 eV by the o p t i c s o f t h e i n s t r u m e n t . Condensed m o l e c u l a r s o l i d s p e c t r a were n o t n e c e s s a r y s i n c e t h e p h o t o e m i s s i o n s t u d i e s h a d a l r e a d y shown t h a t t h e p o l y m e r and c o n d e n s e d m o l e c u l a r s p e c t r a were e q u i v a l e n t . Therefore a comparison o f gas-phase modelm o l e c u l e s p e c t r a w i t h t h a t o f the c o r r e s p o n d i n g polymer s p e c t r a was s u f f i c i e n t .

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Abstract Intermolecular relaxation effects are a central issue in the interpretation of the ultraviolet photoelectron spectroscopy (UPS) of molecular solids. These relaxation effects result in several significant characteristics of UPS valence spectra, i n termolecular relaxation phenomena lead to localized electron molecular-ion states, which are responsible for rigid gas-tosolid molecular spectral energy shifts, spectral line broadening, and dynamic electronic localization effects in aromatic pendant group polymers.

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In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12 Band Structure Calculations and Their Relations to Photoelectron Spectroscopy J E A N - M A R I E A N D R É , J O S E P H D E L H A L L E , and JEAN J A C Q U E S P I R E A U X Facultés Universitaires Notre-Dame de la Paix, 61, rue de Bruxelles, 5000-Namur, Belgium

M e t h o d s F o r The D e t e r m i n a t i o S i n c e t h e f i r s t t h e o r e t i c a l w o r k s o f t h e s i x t i e s (1_) on LCAO t e c h n i q u e s i n p o l y m e r quantum c h e m i s t r y , t h e f i e l d h a s known a r a p i d d e v e l o p m e n t and s t a n d a r d SCF c a l c u l a t i o n s on r e g u l a r p o l y mers a r e now r o u t i n e l y p e r f o r m e d . I n those methods, the t r a n s l a t i o n a l symmetry i s f u l l y e x p l o i t e d ( a n d c o n s e q u e n t l y assumed) i n o r d e r t o r e d u c e t o manageable d i m e n s i o n s t h e f o r m i d a b l e t a s k o f c o m p u t i n g e l e c t r o n i c s t a t e s o f an e x t e n d e d s y s t e m . I n the t h e o r e t i c a l d e s c r i p t i o n o f r e g u l a r p o l y m e r s , the m o n o e l e c t r o n i c l e v e l s ( o r b i t a l energies i n the m o l e c u l a r d e s c r i p t i o n ) a r e r e p r e s e n t e d as a m u l t i v a l u e d f u n c t i o n o f a r e c i p r o c a l wave number d e f i n e d i n t h e i n v e r s e s p a c e d i m e n s i o n . The s e t o f a l l those branches (energy bands) p l o t t e d v e r s u s the r e c i p r o c a l wave number ( k - p o i n t ) i n a w e l l d e f i n e d r e g i o n o f t h e r e c i p r o c a l s p a c e ( f i r s t B r i l l o u i n z o n e ) i s t h e band s t r u c t u r e o f t h e p o l y mers. I n t h e u s u a l t e r m i n o l o g y , we n o t e t h e a n a l o g y b e t w e e n t h e o c c u p i e d l e v e l s and t h e v a l e n c e b a n d s , t h e u n o c c u p i e d l e v e l s and the conduction band. The o n e - d i m e n s i o n a l p e r i o d i c l a t t i c e o f a p o l y m e r c o n s i s t s o f a l a r g e number, 2N + 1 (N-*») o f u n i t c e l l s ^ o f l e n g t h & e a c h containing n u c l e i at p o s i t i o n s % , ^ > •••> ^ ^ ^ c e l l s o r i g i n s j o z , j = 0 , ± 1 , ±2T, ±N, and 2 n electrons d i s t r i b u t e d a l o n g the c h a i n . A s t r i c t e l e c t r o n e u t r a ? i t y o f the cells (2n = + + . . . + Z^) i s necessary. The e l e c t r o n s a r e assumed t o d o u b l y o c c u p y a s e t o f o n e - e l e c t r o n o r b i t a l s , < $ > (k,r), o f B l o c h t y p e w r i t t e n as p e r i o d i c c o m b i n a t i o n s o f a) b a s i s f u n c tions, X (r). A

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where r i s t h e p o s i t i o n v e c t o r measured from an a r b i t r a r y b u t f i x e d o r i g i n , z i s a u n i t v e c t o r i n the d i r e c t i o n o f l a t t i c e p e r i o d i c i t y , P t h e c e n t e r o f t h e b a s i s f u n c t i o n X ( r ) , and k i s a point i n t h e f i r s t B r i l l o u i n zone ( B Z ) , [—TT/£ +T\/CL] , o f t h e 0097-6156/81/0162-0151 $05.00/0 © 1981 American Chemical Society In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

152

PHOTON, ELECTRON, AND ION PROBES

p o l y m e r ; L > t h e l e n g t h o f t h i s B r i l l o u i n z o n e , i s e q u a l t o 2TT/ t e t r a c y a n o q u i n o d i m e t h a n e ( 8 ) and m e t a l c a r b o n y l s . The s a t e l l i t e s h a v e been o b s e r v e d w i t h the l i n e s o f the h e t e r o a t o m s as w e l l as w i t h C i s , w i t h s i m i l a r , b u t n o t i d e n t i c a l , e n e r g y s e p a r a t i o n s and intensities. The e n e r g y s e p a r a t i o n and i n t e n s i t y a r e u n i q u e f o r l i n e s from each atom i n e a c h c h e m i c a l s t a t e . Shakeup s a t e l l i t e s a r e o f t e n more i n t e n s e w i t h p a r a m a g n e t i c i n o r g a n i c i o n s , s u c h as t h o s e o f t h e f i r s t t r a n s i t i o n s e r i e s the r a r e e a r t h s , and the a c t i n i d e s s e r i e s c a n be q u i t e c o m p l e (multiple excited states) possible. Frost, Ishitani, and M c D o w e l l (9) f o r c o p p e r compounds and M a t i e n z o e t a l (10) and Tolman et a l ( J J J f o r n i c k e l compounds have f u r n i s h e d rather c o m p r e h e n s i v e d a t a on s a t e l l i t e e n e r g i e s and i n t e n s i t i e s . The p a r a m a g n e t i c o c t a h e d r a l and t e t r a h e d r a l n i c k e l c o m p l e x e s e x h i b i t shakeup p a t t e r n s , w h i l e the d i a m a g n e t i c s q u a r e p l a n a r ones do not. W i t h c o p p e r t h e p a r a m a g n e t i c c u p r i c s t a t e s have shakeup s a t e l l i t e s w h i l e t h e d i a m a g n e t i c c u p r o u s s t a t e s do n o t . The r u l e i s n o t i n v a r i a b l e , h o w e v e r , and s a t e l l i t e s a r e sometimes o b s e r v e d w i t h d i a m a g n e t i c s p e c i e s , b u t u s u a l l y i n much l o w e r i n t e n s i t y . W i t h the t r a n s i t i o n m e t a l s t h e e x c i t a t i o n mechanism u s u a l l y i n v o l v e s charge t r a n s f e r w i t h the l i g a n d . B r i s k and B a k e r ( 1 2 ) s u m m a r i z e the d i s c u s s i o n s on mechanism t h a t have a p p e a r e d . R a r e e a r t h i o n s a l s o e x h i b i t c o m p l e x shakeup s a t e l l i t e p a t terns. T h a t o f Ce3d i n CeO^ i s e s p e c i a l l y n o t e w o r t h y i n t h e f a c t t h a t two s a t e l l i t e s f o r e a c h o f Ce3d« and C e 3 d ^ * a p p e a r , one o f them more i n t e n s e t h a n t h e p r i m a r y l i n e and s e p a r a t e d from i t by as much as 16 eV ( F i g . 3 ) . A c t i n i d e compounds a l s o e x h i b i t many s a t e l l i t e s in their spectra. Wide use o f t h e s e f e a t u r e s in a n a l y t i c a l work a w a i t s c o l l e c t i o n o f t h e s e s p e c t r a i n a c o m p r e hensive review. 2

Multiplet Splitting. I n t e r a c t i o n o f a core vacancy r e s u l t i n g from the p h o t o e l e c t r i c p r o c e s s w i t h u n p a i r e d e l e c t r o n s i n a v a l e n c e s h e l l induces m u l t i p l e t s p l i t t i n g i n the l i n e s c o r r e s p o n d i n g to the e m i t t e d e l e c t r o n . T h u s , t h e 3s l e v e l i n t h e t r a n s i t i o n metals e x h i b i t r e l a t i v e l y simple s p l i t t i n g s , often o f s e v e r a l e V , s p e c i f i c f o r each c h e m i c a l s t a t e (_13). The 2p l e v e l s a r e s p l i t i n t o m u l t i p l e l i n e s , and t h e e f f e c t o f t h e i r c o n v o l u t i o n i s to w i d e n t h e a p p a r e n t s p l i t o f t h e d o u b l e t ( 1 4 ) . F r o s t , et a l ( 1 5 ) t a b u l a t e the 2p s p l i t t i n g f o r c o b a l t compounds, w h i c h v a r i e s i n t h e r a n g e 1 4 . 6 t o 1 6 . 1 e V . T h e r e s h o u l d be s i m i l a r v a r i a b i l i t y

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

14.

WAGNER

205

Identification of Chemical States by XPS

Perkin-Elmer Corporation

Figure 1. Shakeup line for the C-ls line in polystyrene (2)

Perkin-Elmer Corporation

Figure 2. Examples of shakeup lines in the Cu-2p spectrum (2). Energy separation of the 2p lines is 19.8 eV.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTON, ELECTRON, AND ION PROBES

206

BINDING ENERGY. eV Perkin-Elmer Corporation

Figure 3.

Shakeup spectra in the Ce-3d lines of eerie oxide (2)

*

Figure 4.

9.2eV

Multiplet splitting effects in the Cr-2p doublet

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

14.

WAGNER

207

Identification of Chemical States by XPS

f o r N i , F e , Mn, and C r . Chromate i o n and c h r o m i c o x i d e a r e r e a d i l y d i s t i n g u i s h e d by t h i s e f f e c t ( F i g . 4 ) as w e l l as by t h e A l l o f these f e a t u r e s , the b i n d i n g e n e r g y o f the shakeup s a t e l l i t e s and t h e m u l t i p l e t s p l i t t i n g phenomena, h a v e the advantage t h a t c o r r e c t i o n for s t a t i c charge i s u n n e c e s s a r y . C o r e - T y p e Auger L i n e s , and C h e m i c a l S t a t e P l o t s . The u t i l i t y o f Auger l i n e s i n i d e n t i f i c a t i o n o f c h e m i c a l s t a t e s i s b e i n g r e c o g n i z e d . About h a l f o f the n a t u r a l l y - o c c u r r i n g elements p r e s e n t i n s o l i d s t a t e m a t e r i a l s e x h i b i t A u g e r l i n e s i n XPS s p e c t r a w i t h A l or Mg X - r a d i a t i o n ( F i g . 5 ) . O f t h e s e , t w e n t y two e x h i b i t c o r e - t y p e Auger t r a n s i t i o n s ( f i n a l v a c a n c i e s i n c o r e l e v e l s ) and s i x t e e n have v a l e n c e - t y p e A u g e r t r a n s i t i o n s . Coret y p e Auger groups i n the s p e c t r a o f n e i g h b o r i n g elements are v e r y s i m i l a r , d i f f e r i n g o n l y i n m a g n i t u d e o f l i n e s p a c i n g , and h a v e a t l e a s t one l i n e t h a t i s n a r r o w and i n t e n s e Chemical s h i f t s i n these l i n e s are d i f f e r e n p h o t o e l e c t r o n l i n e s o f th o f t h e most i n t e n s e p h o t o e l e c t r o n l i n e a r e p l o t t e d a g a i n s t t h o s e o f t h e most i n t e n s e or s h a r p A u g e r l i n e f o r compounds o f t h e same e l e m e n t , t h e r e s u l t i n g t w o - d i m e n s i o n a l a r r a y becomes much more useful for i d e n t i f y i n g chemical s t a t e s . An e x a m p l e i s shown i n F i g . 6 for copper. C u p r i c compounds a r e a l l l o c a t e d a t h i g h e r photoelectron binding energies and have identifying shakeup lines. C u p r o u s f o r m s , on t h e o t h e r h a n d , h a v e v i r t u a l l y t h e same p h o t o e l e c t r o n b i n d i n g e n e r g y as e l e m e n t a l c o p p a r , and no s h a k e u p satellite lines. They a r e d i s t i n g u i s h a b l e , h o w e v e r , by t h e A u g e r energy. S i x unknown s a m p l e s c o n t a i n i n g C u , C I , S , and 0 , as w e l l as o t h e r m e t a l s , gave l i n e e n e r g i e s d e n o t e d by t h e c i r c l e s , and i n d i c a t e c o p p e r i s p r o b a b l y p r e s e n t as Cu S, o r p o s s i b l y C u ^ O , b u t c l e a r l y n o t C u C l o r Cu m e t a l . Many o f t h e c h e m i c a l s t a t e p l o t s embodying p r e s e n t k n o w l e d g e o f l i n e e n e r g i e s h a v e b e e n a s s e m b l e d and p u b l i s h e d (17), i n c l u d i n g F , Na, Mg, Cu, Z n , Ge, A s , Se, A g , C d , I n , S n , S b , T e , and I . One f e a t u r e o f t h e s e p l o t s i s o f a d d i t i o n a l i n t e r e s t . The d i a g o n a l g r i d r e p r e s e n t s t h e sum o f t h e A u g e r k i n e t i c e n e r g y and t h e p h o t o e l e c t r o n b i n d i n g e n e r g y , w h i c h we c a l l t h e m o d i f i e d A u g e r P a r a m e t e r (17), : 4x1

f

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In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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ESCA

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i n d i c a t e a component a t 285 eV i n e x c e s s o f t h a t f o r t h e r e p e a t unit. This i s not evident f o r the TiKa data s i n c e the c o n t r i b u t i o n t o t h e t o t a l s i g n a l from t h e v e r y s u r f a c e i s v e r y s m a l l . Such l o w l e v e l s u r f a c e c o n t a m i n a t i o n f o r a f i l m c a n o f c o u r s e more r e a d i l y t o be d e t e c t e d by a n g u l a r d e p e n d e n t s t u d i e s however f o r powder samples where s u c h s t u d i e s a r e n o t a p p l i c a b l e t h e c a p a b i l i t y o f i n t e r r o g a t i n g v e r t i c a l i n h o m o g e n e i t i e s by means o f p h o t o n energy dependent s t u d i e s i s o f g r e a t i m p o r t a n c e . A n g u l a r Dependent S t u d i e s . We have r e c e n t l y p o i n t e d o u t t h e c o n s i d e r a b l e c a r e w h i c h must be e x e r c i s e d i n i n t e r p r e t i n g a n g u l a r ( t a k e o f f a n g l e ) dependent d a t a i n d e l i n e a t i n g u n a m b i g u o u s l y l a t e r a l and v e r t i c a l sample i n h o m o g e n e i t i e s ( 7 ) . The u n r i v a l l e d c a p a b i l i t y o f ESCA i n p r o v i d i n g d a t a on v e r t i c a l i n h o m o g e n e i t i e s has a l r e a d y p r o d u c e d new i n s i g h t s i n t h e s u r f a c e science of p o l y m e r s on b o t h an a c a d e m i c and t e c h n o l o g i c a l f r o n t Before the adven reaction o f a s o l i d surfac addressed. The c o m p l e x i t i e s o f s t r u c t u r e s f o r p o l y m e r s w h i c h a r e p a r t i a l l y a t l e a s t d i c t a t e d by p r o c e s s i n g c a n c l e a r l y l e a d t o c o r r e s p o n d i n g l y complex s u r f a c e s t r u c t u r e s f o r systems w h i c h have undergone r e a c t i o n a t a g a s / s o l i d o r l i q u i d / s o l i d i n t e r f a c e . A t y p i c a l s i t u a t i o n m i g h t be t h a t d e p i c t e d i n F i g u r e 6 . The s u r f a c e , s u b - s u r f a c e and b u l k may have c r y s t a l l i n e and amorphous regions. For a r e a c t i v e species M, for which the a c t i v a t i o n e n e r g y i s s m a l l , r e a c t i o n may o c c u r f o r e v e r y c o l l i s i o n and t h e r e a c t i o n may t h e n o c c u r r a t h e r u n i f o r m l y b o t h l a t e r a l l y and vertically. I f the a c t i v a t i o n energy i s h i g h , however, then b o t h l a t e r a l and v e r t i c a l i n h o m o g e n e i t i e s may o c c u r . The s i t u a t i o n i s a l s o c o m p l i c a t e d by t h e f a c t t h a t t h e c r y s t a l l i n i t y o f a g i v e n p o l y m e r s y s t e m may be s t r o n g l y i n f l u e n c e d by p r o c e s s i n g c o n d i tions. T h u s , u n i a x i a l and b i a x i a l o r i e n t a t i o n o f a g i v e n p o l y m e r s y s t e m may w e l l a l t e r q u i t e d r a s t i c a l l y t h e c r y s t a l l i n i t y o f a g i v e n s y s t e m and t h i s i s i n d i c a t e d s c h e m a t i c a l l y i n F i g u r e 7 . A s we have n o t e d w i t h c a r e f u l c o n t r o l o f a l l t h e v a r i a b l e s a c o m b i n a t i o n o f a n g u l a r and p h o t o n e n e r g y d e p e n d e n t s t u d i e s p r o v i d e s ESCA w i t h u n r i v a l l e d c a p a b i l i t y f o r depth p r o f i l i n g . In appropriate c a s e s i t i s a l s o p o s s i b l e t o s t u d y l a t e r a l i n h o m o g e n e i t i e s and w i t h i m p r o v e d s e n s i t i v i t y s m a l l a r e a a n a l y s i s becomes a d i s t i n c t possibility. T h i s c a n be a c c o m p l i s h e d e i t h e r by means o f t h e s p e c t r o m e t e r l e n s s y s t e m o r by means o f a p p r o p r i a t e s l i t s y s t e m s . A t t h i s s t a g e however i t w o u l d seem t h a t i n t h e n e x t t e n y e a r s we a r e u n l i k e l y _ £ o see s p e c t r o m e t e r s w i t h a s p o t r e s o l u t i o n s i z e o f l e s s t h a n 10 s q . cm. w h i c h i s s t i l l r a t h e r g r o s s compared w i t h SAM. Sample C h a r g i n g . We have shown t h a t sample c h a r g i n g i m p o r t a n t i n f o r m a t i o n l e v e l i n i t s own r i g h t and w i t h t h e b i l i t y o f " t u n i n g " s a m p l e c h a r g i n g by means o f a UV f l o o d the p a r t i c u l a r case o f s t u d i e s i n v o l v i n g a monochromatised

i s an possigun i n source

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

254

PHOTON,

M

ELECTRON,

A N DI O N PROBES

Reactive species M

Diffusion in amorphou (?) Low activation energy:- uniform reaction at surface (ii) High

" . - diffusion controlled, non uniform reactivity of various surface and subsurface regions. Figure 6.

Figure 7.

Surface reactions of polymers

Fringed micelle structure of polymers and orientation effects on bulk morphology

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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t h e r e w i l l u n d o u b t e d l y be an i n c r e a s i n g a w a r e n e s s o f t h i s i m p o r t a n t i n f o r m a t i o n l e v e l w h i c h i s t r a d i t i o n a l l y t h r o w n away ( 8 ) . I n t h e t h r e e s e c t i o n s b e l o w we c o n s i d e r i m p o r t a n t a r e a s o f a p p l i c a t i o n s o f ESCA i n ; m i g r a t i o n phenomena, i n t h e s y n t h e s i s o f p o l y m e r s u r f a c e s by means o f p l a s m a t e c h n i q u e s and i n r e a c t i o n s i n i t i a t e d at the surface o f c o t t o n f i b r i l s i n the n i t r a t i o n o f cellulose. M i g r a t i o n Phenomena (9^) A s we have i n d i c a t e d s c h e m a t i c a l l y i n F i g u r e 2 t h e m i g r a t i o n and s e g r e g a t i o n o f s m a l l m o l e c u l e s a t p o l y m e r s u r f a c e s c a n b e m o n i t o r e d by i n t e n s i t y changes o f a p p r o p r i a t e c o r e l e v e l s as a f u n c t i o n o f take o f f angle. A particularly intriguing applicat i o n o f t h i s genre i n v o l v e s the m i g r a t i o n o f the low m o l e c u l a r w e i g h t s i l i c o n e m a t e r i a l u s e d as a r e l e a s e a g e n t i n d o u b l e s i d e d S c o t c h t a p e commonly use ments. A p r i o r i m i g r a t i o n o f low m o l e c u l a r w e i g h t m a t e r i a l c o u l d c o n c e i v a b l y o c c u r by two d i s t i n c t mechanisms and t h i s i s i n d i c a t e d s c h e m a t i c a l l y i n F i g u r e 8 . On t h e l e f t hand s i d e m i g r a t i o n from one s u r f a c e o f a p o l y m e r t o t h e o t h e r i s by b u l k m i g r a t i o n i n v o l v i n g permeation through the b u l k . The a l t e r n a t i v e i n v o l v e s m i g r a t i o n a l o n g s u r f a c e s as i n d i c a t e d s c h e m a t i c a l l y on t h e r i g h t hand s i d e o f t h e f i g u r e . A n ESCA e x a m i n a t i o n o f a f r e s h l y e x p o s e d s u r f a c e o f d o u b l e s i d e d S c o t c h t a p e ( 3 M ' s company e l e c t r i c a l i n s u l a t i n g t a p e ) r e v e a l s a f a i r l y i n t e n s e s i g n a l from S i 2 and 0 1 s l e v e l s a p p r o p r i a t e to s i l i c o n e type m a t e r i a l . To d e m o n s t r a t e t h e m i g r a t i o n o f t h i s s i l i c o n e m a t e r i a l a p i e c e o f t a p e was a t t a c h e d t o a s t r i p o f p o l y m e r and t h e r e s u l t s f o r h i g h and l o w d e n s i t y polyethylene samples (HDPE LDPE) a r e shown i n F i g u r e 9 . By t a k i n g s a m p l e s f r o m d i f f e r e n t s e c t i o n s o f t h e p o l y m e r s t r i p a f t e r a p e r i o d o f 10 days a t a m b i e n t t e m p e r a t u r e two f e a t u r e s a r e e v i d e n t . F i r s t l y LDPE h a s a d i f f e r e n t b e h a v i o r f r o m HDPE. S e c o n d l y b o t h p o l y m e r f i l m s show evidence that the s i l i c o n e w i l l migrate along the s u r f a c e . This i s n o t e n t i r e l y u n e x p e c t e d on t h e b a s i s o f t h e s u r f a c e e n e r g e t i c s and t h e l i k e l y d e g r e e o f c r y s t a l l i n i t y e t c . o f t h e s u r f a c e s o f h i g h and l o w d e n s i t y p o l y e t h y l e n e . I n o r d e r t o d i f f e r e n t i a t e between m i g r a t i o n a l o n g a s u r f a c e v e r s u s p e r m e a t i o n t h r o u g h t h e b u l k as mechanisms f o r s i l i c o n e t o move from one s u r f a c e t o a n o t h e r we h a v e c o n s t r u c t e d t h e a p p a r a t u s shown i n F i g u r e 1 0 . By m o u n t i n g p o l y m e r f i l m s i n a c l a m p i n g r i n g and by m o n i t o r i n g d i f f e r e n t portions o f t h e f i l m i t becomes p o s s i b l e to d i f f e r e n t i a t e between the main a l t e r n a t i v e s for mig r a t i o n mechanisms and t h i s i s i n d i c a t e d s c h e m a t i c a l l y i n t h e figure. The c o r e l e v e l d a t a f o r two o f t h e p o s i t i o n s o f i n t e r e s t f o r a LDPE f i l m o f ^ 1 0 0 u t h i c k n e s s a r e shown i n F i g u r e 1 1 . C o n s i d e r i n g f i r s t l y t h e room t e m p e r a t u r e d a t a t h e i n t e n s i t i e s f o r t h e S i 0

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Prototype system Low MWt silicone on HDPE and LDPE

Figure 8.

Migration phenomena: possible extremes for migration of low-molecular weight materials.

HDPE

tape LDPE

Figure 9.

The C-ls, O-ls, and Si-2p core levels for strips of LDPE film with Scotch tape attached (diffusion studies, 10 days RT)

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwigh ACS Symposium Series; American Chemical Society: Washington, DC, 19

POLYMER STUDIED-

escapes vessel

if it escapes vessel

Diffusion studies apparatus and its application (1:1 scale diagram)

diffusion through lateral migration

migration around edge of polymer

Figure 10.

Shows extent of Shows extent to which

MODES OF SAMPLING

CROSS SECTION

Sample to show extent of lateral

MEDIUM

DIFFUSION

PLAN VIEW

polymer

Figure 12.

Comparison of C-ls and Si-2p levels for HDPE and LDPE samples mounted in diffusion study apparatus

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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and 0 1 s s i g n a l s a r e somewhat s i m i l a r f o r b o t h p o s i t i o n s and i n d i c a t e t h a t m i g r a t i o n a l o n g the s u r f a c e i s d r a s t i c a l l y c u r t a i l ed by t h e c l a m p i n g r i n g a r r a n g e m e n t . A f t e r 11 d a y s a t 75 C however t h e s i t u a t i o n i s s u b s t a n t i a l l y d i f f e r e n t t h e m i g r a t i o n through the b u l k b e i n g s u b s t a n t i a l l y f a s t e r than along the s u r face. The d i f f e r e n c e b e t w e e n HDPE and LDPE i s s t i l l evident however f o r t h e b u l k m i g r a t i o n and t h i s i s c l e a r from t h e d a t a g i v e n i n F i g u r e 12. These d a t a show t h e p o t e n t i a l f o r s t u d y i n g t h e mode o f m i g r a t i o n o f s m a l l m o l e c u l e s b o t h a l o n g and t h r o u g h s u r f a c e s and a l s o p o i n t t o t h e d a n g e r o f l o n g t e r m e x p o s u r e o f samples t o S c o t c h t a p e as m i g h t be u s e d f o r example i n s e c u r i n g samples f o r t r a n s p o r t i n g purposes. The P l a s m a S y n t h e s i s O f U l t r a T h i n F l u o r o p o l y m e r F i l m s The s y n t h e s i s o f u l t r n i q u e s i s an a r e a o f c o n s i d e r a b l v i e w p o i n t s ( H ) , 11). Thus t h e c a p a b i l i t y o f t a k i n g b u l k p o l y m e r s o f d e s i r a b l e b u l k p r o p e r t i e s and s p e c i f i c a l l y m o d i f y i n g t h e s u r face to a l t e r the c h e m i c a l , p h y s i c a l , m e c h a n i c a l or e l e c t r i c a l p r o p e r t i e s h a s much t o commend i t and f i e l d s o f a p p l i c a t i o n w h i c h have a l r e a d y r e c e i v e d a t t e n t i o n i n c l u d e g r a f t i n g p o l y a c r y l i c a c i d o n t o p o l y e s t e r f i b r e t o i m p r o v e t h e c o m f o r t f a c t o r and d e p o s i t i o n o f low f r i c t i o n c o a t i n g f l u o r o p o l y m e r f i l m s o n t o e l a s t o m e r s e a l s f o r use i n the o i l d r i l l i n g i n d u s t r y . Plasma d e p o s i t i o n o f u l t r a t h i n polymer f i l m s onto metals i s o f i n t e r e s t f o r b i o m o n i t o r i n g e l e c t r o d e s and p l a s m a c o a t i n g s a r e a l s o u n d e r i n v e s t i g a t i o n f o r laser fusion targets. Plasma polymers p r e p a r e d from h e x a m e t h y l d i s i l o x a n e have b e e n shown t o be amongst t h e most b i o c o m p a t i b l e o f p o l y m e r i c f i l m s w h i l s t the d e p o s i t i o n o f v e r y t h i n u n i f o r m p i n h o l e f r e e f i l m s o f u n u s u a l e l e c t r i c a l p r o p e r t i e s makes p l a s m a polymer f i l m s o f i n t e r e s t i n device a p p l i c a t i o n s . The s p e c i f i c m o d i f i c a t i o n o f p o l y m e r s u r f a c e s by " c o o l " p l a s m a s i s a l s o an a r e a of considerable current interest. Thus d e t a i l e d k i n e t i c s t u d i e s have b e e n made o f d i r e c t and r a d i a t i v e e n e r g y t r a n s f e r t o p o l y m e r s u r f a c e s by means o f i n e r t gas p l a s m a s , a t o p i c o f i n t e r e s t i n t h e s u r f a c e c r o s s - l i n k i n g o f p o l y m e r s and r e m o v i n g weak b o u n d a r y layers for adhesive bonding. The s u r f a c e f u n c t i o n a l i s a t i o n o f p o l y m e r s c a n a l s o be a c c o m p l i s h e d i n a c o s t e f f e c t i v e manner by means o f p l a s m a t e c h n i q u e s . The m o t i v a t i o n f o r s t u d y i n g t h e f u n d a m e n t a l s o f p l a s m a p o l y m e r i z a t i o n s and m o d i f i c a t i o n s o f s u r f a c e s i s t h e r e f o r e n o t d i f f i c u l t t o u n d e r s t a n d and we c o n s i d e r h e r e one a s p e c t o f t h e t o p i c , t h e p l a s m a p o l y m e r i z a t i o n o f s i m p l e fluoropolymers. The e x c i t a t i o n o f a p l a s m a i n a t y p i c a l o r g a n i c m o l e c u l e g i v e s r i s e t o a number o f r a d i c a l c a t i o n s , and e x c i t e d s t a t e s and a n i o n r a d i c a l s a r i s i n g from e l e c t r o n a t t a c h m e n t . I t i s not s u r p r i s i n g t h e r e f o r e t h a t t r a n s f o r m a t i o n s i n v o l v i n g such s p e c i e s and r e a c t i o n s w i t h t h e o r i g i n a l m o l e c u l e e i t h e r i n t h e gas p h a s e

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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o r at the g a s - s o l i d i n t e r f a c e g i v e s r i s e t o a r e a c t i o n pathway which i s t y p i c a l l y q u i t e complex. F o r v i n y l monomers i t may b e shown t h a t u n d e r a p p r o p r i a t e c o n d i t i o n s p l a s m a i n d u c e d p o l y m e r i z a t i o n s may accompany t h e more g e n e r a l p r o c e s s o f p l a s m a p o l y m e r i z a t i o n , the former g i v i n g r i s e to l i n e a r systems, the l a t t e r cross-linked products. I n t h e g l o w r e g i o n o f t h e p l a s m a where t h e i o n d e n s i t y i s h i g h e s t polymer d e p o s i t i o n a r i s e s from the competi t i v e p r o c e s s o f p o l y m e r i z a t i o n and a b l a t i o n , t h e s o - c a l l e d CAP scheme ( 1 0 ) and t h e g r e a t v e r s a t i l i t y o f t h e t e c h n i q u e i n b e i n g a b l e t o p r o d u c e t h i n f i l m s f r o m v i r t u a l l y any o r g a n i c p r e c u r s o r makes t h e s t u d y o f t h e p r o c e s s o f d e p o s i t i o n and t h e s t r u c t u r e o f t h e a c c o m p a n y i n g f i l m s one o f g r e a t i n t e r e s t . The f a c t t h a t t h e f i l m s o f i n t e r e s t a r e t h i n ( g e n e r a l l y i n t h e r a n g e 5 - 5000X) and t h e r e f o r e need t o be s t u d i e d i n s i t u and a r e i n s o l u b l e r e n d e r s ESCA t h e t e c h n i q u e o f c h o i c e f o r s t u d y i n g d e t a i l s o f s t r u c t u r e and bonding. I n d e e d i t i s t r u e t o say t h a t t h e m e c h a n i s t i c s t u d y o f the f o r m a t i o n o f such f i l m s i n c e t h e a d v e n t o f ESCA I n r e c e n t p a p e r s we have shown how ESCA may be u s e d t o establish r e l a t i o n s h i p between the s t r u c t u r e o f the initial m o l e c u l a r s y s t e m i n w h i c h t h e d i s c h a r g e i s e x c i t e d and t h a t o f t h e p o l y m e r d e p o s i t e d i n e i t h e r g l o w o r n o n - g l o w r e a t i o n s (1_2). The o b j e c t i v e s o f t h e p r e s e n t programme o f w o r k on p l a s m a p o l y m e r s may b e summarized as f o l l o w s : (i) To e s t a b l i s h t h e o v e r a l l s t o i c h i o m e t r y and s t r u c t u r a l f e a t u r e s present i n a polymer f i l m . (ii) To s t u d y the v a r i a t i o n i n ( i ) as a f u n c t i o n o f g e o m e t r i c f a c t o r s , power, p r e s s u r e e t c . (iii) To s t u d y r a t e s o f d e p o s i t i o n as a f u n c t i o n o f t h e gas and t h e s i t e o f d e p o s i t i o n . F o r a v a r i e t y o f r e a s o n s t h e work r e p o r t e d h e r e p e r t a i n s t o i n d u c t i v e l y c o u p l e d RF p l a s m a s i n t h e p r e s s u r e r a n g e 50 - 2 0 0 u . Such e x p e r i m e n t s a r e p a r t i c u l a r l y e a s y t o c a r r y o u t and c l o s e c o n t r o l c a n be m a i n t a i n e d on t h e i m p o r t a n t v a r i a b l e s s u c h as p r e s s u r e f l o w r a t e , power i n p u t e t c . Y a s u d a ( 1 0 ) has p o i n t e d o u t the i m p o r t a n c e o f W/FM (where W i s t h e power i n p u t , F i s t h e f l o w r a t e and M the m o l e c u l a r w e i g h t ) i n d e t e r m i n i n g d e p o s i t i o n r a t e and n a t u r e o f t h e m a t e r i a l ( v i z . p o w d e r , powdery f i l m , f i l m , o i l ) . Under t h e c o n d i t i o n s o f work d e s c r i b e d h e r e p o l y m e r s were found t o d e p o s i t as u n i f o r m f i l m s b o t h on g o l d , p y r e x and s i l i c a substrates. T h r e e d i s t i n c t a p p a r a t u s ' s have been employed i n t h i s work. A schematic o f a t y p i c a l free s t a n d i n g r e a c t o r f o r the d e p o s i t i o n of f i l m s which are subsequently t r a n s f e r r e d to the s p e c t r o m e t e r i s shown i n F i g u r e 1 3 , w h i l s t t h e r e a c t o r u s e d f o r i n s i t u d e p o s i t i o n o f p o l y m e r f i l m s w h i c h may t h e n be d i r e c t l y i n s e r t e d i n t o t h e s p e c t r o m e t e r h a s b e e n d e s c r i b e d i n some d e t a i l p r e v i o u s l y (11). I n o r d e r t o s t u d y the dependence o f p o l y m e r s t r u c t u r e and r a t e o f d e p o s i t i o n as a f u n c t i o n o f d e p o s i t i o n s i t e a s p e c i a l l o n g r e a c t o r has been c o n s t r u c t e d and t h i s i s shown s c h e m a t i c a l l y i n F i g u r e 14.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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TEFLON VALVES

PUMPING

CAJON ULTRA TORR

VITON 0 RING SEALS

COUPLINGS 1"dia.

REACTOR * 1

REACTOR * 2

Figure 13. Apparatus for studying RF plasma polymers

~i 8cm h

H 0

| 10

-

coil only In turns -soft copper)

I

I

25

45

I

I 60

80

I 95

Figure 14. Apparatus for studying site dependence of structure of plasma polymers

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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I t i s known t h a t p l a s m a p o l y m e r s o f t e n c o n t a i n u n p a i r e d s p i n s and as s u c h may r e a c t on e x p o s u r e t o a t m o s p h e r e o x y g e n t o produce s u r f a c e o x i d a t i v e f u n c t i o n a l i z a t i o n . I n g e n e r a l t h i s has n o t p r o v e d t o be o f any m a j o r s i g n i f i c a n c e as f a r as t h e f l u o r o p o l y m e r s s t u d i e d i n t h i s work a r e c o n c e r n e d and t o e m p h a s i z e t h i s p o i n t F i g u r e 15 shows a w i d e s c a n ESCA s p e c t r u m o f a t h i n p l a s m a p o l y m e r f i l m p r o d u c e d from p e r f l u o r o b e n z e n e . The s t o i c h i o m e t r y o f t h i s f i l m i s c l o s e t o t h a t o f t h e i n i t i a l "monomer" and by g o i n g t o a h i g h t a k e o f f a n g l e we c a n d e t e c t a v e r y l o w l e v e l o f o x y g e n s i g n a l f o r t h i s f i l m p r o d u c e d i n an a p p a r a t u s as i n F i g u r e 13. A s w e l l as h a v i n g i n t e r e s t i n g c h e m i c a l , p h y s i c a l , e l e c t r i c a l and m e c h a n i c a l p r o p e r t i e s f l u o r o p o l y m e r s p r o d u c e d by p l a s m a means a r e p a r t i c u l a r l y s u i t a b l e f o r s t u d y by ESCA n o t t h e l e a s t b e c a u s e o f the v e r y d i s t i n c t i v e n a t u r e o f the C i s l e v e l s i n such systems. F i g u r e 16 d e p i c t s t y p i c a l b i n d i n g e n e r g y r a n g e s f o r a v a r i e t y o f s t r u c t u r a l features whic and w i t h a b a c k g r o u n d o s p e c t r a for plasma polymer systems. A p a r t i c u l a r l y s t r i k i n g example o f t h i s g e n r e i s p r o v i d e d by the d a t a g i v e n i n F i g u r e 17. The s t o i c h i o m e t r y o f f i l m s c a n b e d e t e r m i n e d from t h e i n t e g r a t e d C l s : F l s i n t e n s i t y r a t i o s and f r o m t h e components o f t h e C i s l e v e l s . From t h i s d a t a i t i s c l e a r t h a t plasma p o l y m e r i z a t i o n i n v o l v e s s u b s t a n t i a l rearrangement s i n c e t r i f l u o r o m e t h y 1 groups form a s i g n i f i c a n t f e a t u r e o f the s p e c t r a . F o r p e r f l u o r o c y c l o h e x a n e , n o r m a l l y c o n s i d e r e d an e x t r e m e l y i n e r t f l u o r o c a r b o n , the f l u o r o p o l y m e r e x h i b i t s a l a r g e p e r c e n t a g e o f p e r f l u o r o e t h y l g r o u p s and has a s t o i c h i o m e t r y l o w e r i n f l u o r i n e than the s t a r t i n g m a t e r i a l . W i t h the advent o f ESCA as a s t r u c t u r a l t o o l i t has b e e n shown f o r t h e f i r s t t i m e t h a t t h e d e p o s i t i o n i n the glow r e g i o n o f a plasma can produce w e l l d e f i n e d p o l y m e r i c p r o d u c t s o v e r a r a n g e o f p r e s s u r e and power i n p u t r a n g e s (12) . The n a t u r e o f t h e c o m p e t i t i v e p r o c e s s o f p o l y m e r i z a t i o n and d e p o s i t i o n i s s u c h t h a t we m i g h t a n t i c i p a t e t h a t a t t h e edges and o u t s i d e o f the glow r e g i o n l o n g e r l i v e d r e a c t i v e s p e c i e s m i g h t w e l l l e a d t o the f o r m a t i o n o f d i f f e r e n t p o l y m e r i n r e g i o n s r e m o t e f r o m t h e c o i l r e g i o n i n an i n d u c t i v e l y c o u p l e d p l a s m a . Indeed i n t h e c a s e o f c o n v e n t i o n a l l y p o l y m e r i z a b l e "monomers" s u c h as t h e f l u o r i n a t e d a l k e n e s t h e p o l y m e r i z a t i o n mechanism i n t h e n o n - g l o w r e g i o n may w e l l be d o m i n a t e d by c o n v e n t i o n a l a d d i t i o n p o l y m e r ization: t h e so c a l l e d p l a s m a i n i t i a t e d p o l y m e r i z a t i o n . There are two i m p o r t a n t q u e s t i o n s w h i c h may r e a d i l y b e a d d r e s s e d by ESCA. F i r s t l y , what i s t h e r a t e o f d e p o s i t i o n o f t h e p o l y m e r f i l m a t a g i v e n s i t e i n a p l a s m a r e a c t o r and s e c o n d l y how d o e s t h e s t r u c t u r e depend on t h e s i t e o f d e p o s i t i o n ? To i l l u s t r a t e t h e g r e a t power o f t h e t e c h n i q u e i n a n s w e r i n g t h e s e q u e s t i o n s we c o n s i d e r h e r e a r e c e n t d e t a i l e d i n v e s t i g a t i o n o f t h e i n d u c t i v e l y coupled plasma p o l y m e r i z a t i o n o f pentafluorobenzene (13) .

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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C-CF CF-C-CF CH CC 2

CFtr-^TT*CF

297

CF

295

3

£F

293

291

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2

289

287

285

Figure 16. The C-ls core binding energies forfluorocarbonstructural features

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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C i s c o r e l e v e l s p e c t r a as a f u n c t i o n o f two t a k e o f f a n g l e s f o r the p l a s m a p o l y m e r i z a t i o n i n the g l o w r e g i o n o f a free s t a n d i n g r e a c t o r a r e shown i n F i g u r e 1 8 , a t a p r e s s u r e o f 200u and power l o a d i n g o f 10 w a t t s . The s p e c t r a show c o n s i d e r a b l e f i n e s t r u c t u r e and c o m p a r i s o n w i t h d a t a s u c h as t h a t i n F i g u r e 16 allows a s t r a i g h t f o r w a r d assignment. The m a j o r components o f t h e l i n e p r o f i l e c o n s i s t s o f two p e a k s c e n t e r e d ^ 2 8 6 . 7 eV and 2 8 8 . 7 eV c o r r e s p o n d i n g t o T T * s h a k e up satellite of the component centered ^ 2 8 8 . 7 eV t h u s i n d i c a t i n g t h e p r e s e n c e o f a c o n j u g a t e d system. The s m a l l component a t 285 eV a l m o s t c e r t a i n l y c o r r e s ponds to a low l e v e l o f h y d r o c a r b o n c o n t a m i n a t i o n s i n c e the r e l a t i v e i n t e n s i t y i n c r e a s e s s l i g h t l y on g o i n g t o a h i g h e r t a k e off angle. Within experimenta t a k e o f f angles are th c o n t e n t t h a n the s t a r t i n g m a t e r i a l . The p r e s e n c e o f C F ^ and C F ^ s t r u c t u r a l f e a t u r e s p r o v i d e s s t r o n g e v i d e n c e f o r the e x t e n s i v e m o l e c u l a r rearrangement accompanying the p o l y m e r i z a t i o n r e a c t i o n . The s t r u c t u r e and c o m p o s i t i o n o f t h e p o l y m e r d e p o s i t e d i n t h e g l o w r e g i o n r e m a i n s e s s e n t i a l l y c o n s t a n t as a f u n c t i o n o f power and p r e s s u r e o v e r a c o n s i d e r a b l e r a n g e o f t y p i c a l o p e r a t i n g parameters. T h i s F i g u r e 19 shows t h e component a n a l y s i s f o r f i l m s d e p o s i t e d on g o l d s u b s t r a t e s i n t h e g l o w r e g i o n a t a fixed p r e s s u r e o f 200u and power i n p u t s o f 1 0 , 20 and 30 w a t t s . W i t h a k n o w l e d g e o f mean f r e e p a t h s as a f u n c t i o n o f k i n e t i c e n e r g y i t becomes p o s s i b l e t o s t u d y t h e r a t e o f f i l m d e p o s i t i o n as a f u n c t i o n o f the o p e r a t i n g parameters. T y p i c a l d a t a f o r the i n s i t u d e p o s i t i o n o f polymer i n a s m a l l r e a c t o r a t t a c h e d to the s p e c t r o m e t e r a r e g i v e n i n F i g u r e 2 0 . By m o n i t o r i n g t h e s u b s t r a t e A u ^ „ , l e v e l s (MgK . ^ r a d i a t i o n ) t h e r a t e s o f d e p o s i t i o n may be s t u d i e d and t h e u n i f o r m n a t u r e o f t h e d e p o s i t i o n i s e v i d e n t f r o m this data. F o r a f i v e f o l d i n c r e a s e i n power t h e d e p o s i t i o n r a t e c h a n g e s f r o m ^ 2A p e r s e c . t o per s e c . i n d i c a t i n g t h a t the r a t e o f d e p o s i t i o n i s a p p r o a c h i n g t h e maximum a t t h e p r e s s u r e and f l o w r a t e employed f o r t h e s e e x p e r i m e n t s . We now t u r n t o t h e i m p o r t a n t q u e s t i o n o f t h e s t r u c t u r e o f t h e p o l y m e r as a f u n c t i o n o f s i t e d e p o s i t i o n . Employing the r e a c t o r c o n f i g u r a t i o n d e p i c t e d i n F i g u r e 14, the polymer d e p o s i t e d at the r e a c t o r w a l l and a t t h e c e n t e r o f t h e r e a c t o r h a s b e e n i n v e s t i g a t e d as a f u n c t i o n o f p o s i t i o n as i n d i c a t e d . The g l o w r e g i o n f o r a p l a s m a e x c i t e d a t 200u and 10 w a t t s i n p u t power e x t e n d s f o r a r e g i o n ^ 1 5 cms. i n from o f and ^ 1 0 cm. t o t h e r e a r o f t h e c o i l region. F i g u r e 21 shows t h e F l s , C i s and F 2 s l e v e l s f o r the polymer d e p o s i t e d i n the glow r e g i o n i n f r o n t o f the c o i l system. The s p e c t r a a r e e s s e n t i a l l y i d e n t i c a l t o t h o s e i n F i g u r e 1 8 . The c o r e l e v e l s p e c t r a f o r t h e f i l m d e p o s i t e d a t t h e edge o f t h e g l o w r e g i o n 45 cms. from t h e f r o n t o f t h e r e a c t o r a r e d i s t i n c t i v e l y n >

2

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297 295 293 29 Binding energy ev

Figure 18.

The C-ls core levels as a function of take-off angle for RF plasma polymer from pentafluorobenzene (10 W, 200 fi)

H RF

plasma polymerization

40Q"CF

n

35- A CF - C 30%

C:F stoichiometry 25-

contribution

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to C i profile

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1:072

1 072

s

20-

1 5

.

micro analysis 1 :071

-» CF3 " * IT-^Tf* shake up IQF) 20

30

power Iwatts)

Figure 19. Component analysis of structural features in fluoropolymer films from RF plasmas excited in pentafluorobenzene (free-standing reactor (200p))

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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5

10

15

5

10

15

20 tfsec)

25

30

20

25

30

35

tlsec)

Figure 20. Rates of deposition of plasma polymerS from pentafluorobenzene

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Figure 21. The F-ls, C-ls, and F-2s levels for fluoropolymer produced from pentafluorobenzene (site of deposition 10 cm from input end of reactor in Figure 14)

Figure 22. The F-ls, C-ls, and F-2s levels for fluoropolymer produced from pentafluorobenzene (site of deposition 45 cm from input end of reactor in Figure 14)

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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d i f f e r e n t as i s a p p a r e n t f r o m a c o m p a r i s o n o f F i g u r e 22 a n d F i g u r e 21. C l e a r l y t h e c o n t r i b u t i o n from C F ^ s t r u c t u r a l f e a t u r e s h a s s i g n i f i c a n t l y i n c r e a s e d and t h i s becomes more p r o n o u n c e d f o r f i l m s d e p o s i t e d i n t h e non g l o w r e g i o n 60 cms. f r o m t h e f r o n t o f t h e r e a c t o r ( F i g u r e 2 3 ) . C F ^ s t r u c t u r a l f e a t u r e s now d o m i n a t e t h e s p e c t r a and F i g u r e 24 shows how t h e d e r i v e d s t o i c h i o m e t r y o f t h e p o l y m e r f i l m depends on s i t e o f d e p o s i t i o n . I t may a l s o b e shown t h a t the r a t e o f d e p o s i t i o n i n the glow r e g i o n i s extremely r a p i d compared w i t h t h e non g l o w r e g i o n s . With the type o f i n f o r m a t i o n p r o v i d e d by t h e s e ESCA s t u d i e s t h e s t o i c h i o m e t r y and m a i n s t r u c t u r a l f e a t u r e s f o r a g i v e n p o l y m e r f i l m d e r i v e d from a g i v e n s t a r t i n g m a t e r i a l may b e s e l e c t e d f o r a g i v e n a p p l i c a t i o n . An ESCA I n v e s t i g a t i o n O f The S u r f a c e C h e m i s t r y O f The N i t r a t i o n And D e n i t r a t i o n O f C e l l u l o s e M a t e r i a l s Introduction. Th p a r t i c u l a r l y i n the for g chequer ed h i s t o r y (_14). I n d e e d t h e d i s c o v e r y i n 1833 by B r a c c o n n o t ( 1 5 ) that the r e a c t i o n o f n i t r i c a c i d w i t h c o t t o n produced inflammable m a t e r i a l s and t h e s u b s e q u e n t p a t e n t g r a n t e d t o S c h o n b e i n (16) i n 1846 s p e c i f y i n g t h e p r o d u c t i o n o f n i t r o c e l l u l o s e s from c o t t o n and m i x e d a c i d s c a n be s a i d t o h a v e m a t e r i a l l y changed t h e c o u r s e o f history. A f t e r the f i r s t (unscheduled) l a r g e s c a l e e x p l o s i o n i n 1 8 4 7 , work on t h e t e c h n i c a l p r o d u c t i o n o f n i t r o c e l l u l o s e s was somewhat r e s t r a i n e d , a f t e r t h i s m i s h a p however t h e p o s s i b i l i t y o f m o d i f y i n g b u r n r a t e f o r p r o p u l s i o n p u r p o s e s was r e c o g n i z e d and by 1867 t h e b a s i s o f t h e p r o d u c t i o n o f s m o k e l e s s p r o p e l l a n t s f o r g u n s , r i f l e s and s h e l l s was e s t a b l i s h e d and m a t e r i a l l y changed the course o f h i s t o r y n o t o n l y i n the m i l i t a r y sense b u t a l s o i n the p i o n e e r i n g days o f the w i l d w e s t . The i n n o v a t i v e u s e o f camphor as a p l a s t i c i z e r f o r n i t r o c e l l u l o s e b y P a r k e s i n 1862 (_17) i n t h e p r o d u c t i o n o f c e l l u l o i d r e p r e s e n t s an i m p o r t a n t l a n d m a r k i n t h e emergence o f p o l y m e r s c i e n c e and t h e o b s e r v a t i o n o f t h e s o l u b i l i t y o f n i t r o c e l l u l o s e s and t h e p r o d u c t i o n o f C o l l o d i o n and t h i n f i l m s u n d o u b t e d l y a c c e l e r a t e d t h e d e v e l o p m e n t o f p h o t o g r a p h y as we now know i t . A s an amusing s i d e l i g h t the p r o d u c t i o n o f c e l l u l o i d a l l o w e d the w i d e r d e v e l o p m e n t o f s n o o k e r and b i l l i a r d s . The p l a s t i c i z i n g e f f e c t o f camphor on n i t r o c e l l u l o s e u n d o u b t e d l y p r o v i d e d c o n s i d e r a b l e i m p e t u s t o t h e d e v e l o p m e n t o f d o u b l e and t r i p l e b a s e d propellants b a s e d on n i t r o g l y c e r i n e / n i t r o c e l l u l o s e f o r m u l a t i o n s . The f o u n d a t i o n s o f t h e p r o d u c t i o n methods u s e d f o r n i t r o c e l l u l o s e s ( F i g u r e 25) were l a r g e l y l a i d down i n t h e e a r l y t w e n t i e t h c e n t u r y and d e s p i t e t h e t e c h n o l o g i c a l , m i l i t a r y and academic i m p o r t a n c e o f t h e w h o l e f i e l d and d e s p i t e a v o l u m i n o u s l i t e r a t u r e much o f i t c l a s s i f i e d t h e r e a r e a number o f unanswered q u e s t i o n s . This i s p a r t i c u l a r l y so when t h e i m p o r t a n t q u e s t i o n o f s u r f a c e c h e m i s t r y is addressed. Thus t h e i n i t i a l i n t e r a c t i o n o f c e l l u l o s e f i b r i l s w i t h n i t r a t i n g media, the burn i n i t i a t i o n , the s t a b i l i z a t i o n ,

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Cis

Figure 23. The F-ls, C-ls, for fluoropolymer produced from pentafluorobenzene (site of deposition 60 cm from input end of reactor in Figure 14)

18

H 1-6

p*]

RF plasma 10 watts 200 u

U 1-2 10 Average C/F Stoichiometry

0

.

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8

0-6

-];

. . . . \ stoichiometry of monomer

11 11111 i, 11

0* coil region 0-2

+. flow direction 0

10

20

30

£0

50

60

position in reactor (at wall)

Figure 24.

Average carbon to fluorine stoichiometry as a function of site of deposition for fluoropolymers produced from pentafluorobenzene

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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r e p r e s e n t a few amongst many o f t h e i m p o r t a n t a s p e c t s o f t h e n i t r o c e l l u l o s e f i e l d w h i c h depend on s u r f a c e phenomena. The w i d e r a n g i n g n a t u r e o f b o t h t h e t e c h n o l o g i c a l and a c a d e m i c i n v e s t i g a t i o n s o f t h e c e l l u l o s e and n i t r o c e l l u l o s e f i e l d o v e r t h e p a s t h u n d r e d y e a r s o r so p o s e s c e r t a i n d i f f i c u l t i e s s i n c e r e l e v a n t d a t a i s w i d e l y d i s p e r s e d o v e r a number o f d i s c i p l i n e s . The most c o m p l e t e p u b l i s h e d s u r v e y ( 1 4 ) i s now some 30 y e a r s o l d and p r e - d a t e s most o f t h e r e l e v a n t d e v e l o p m e n t s i n b o t h s u r f a c e c h e m i s t r y and m e c h a n i s t i c o r g a n i c c h e m i s t r y w h i c h a r e e s s e n t i a l to the d e t a i l e d u n d e r s t a n d i n g o f heterogeneous p r o c e s s e s at the surfaces o f c e l l u l o s e based m a t e r i a l s . The n i t r a t i o n o f c e l l u l o s e i n t h e form o f c o t t o n l i n t e r s i s u s u a l l y i n m i x e d a c i d and on a t h r e e component d i a g r a m t h e zone o f t e c h n i c a l n i t r a t i o n i s d e l i n e a t e d by c o m p o s i t i o n s i n t h e r a n g e H N 0 17-27%, H S 0 66-50% and H O 17-23% ( 1 4 ) . A l t h o u g h n i t r a t i o n s c a n be c o n v e n i e n t l y e f f e c t e d on t h e l a b o r a t o r y s c a l e w i t h n i t r i c p h o s p h o r i c mixe n o r m a l l y p r e c l u d e the scale. N i t r i c a c i d - water mixes a r e not used s i n c e n i t r a t i o n i s u n e v e n and d e g r a d a t i o n and g e l a t i n i z a t i o n o f p r o d u c t a r i s e a t t h e h i g h percentage n i t r i c mixes r e q u i r e d f o r r e a c t i o n . Technical n i t r a t i o n s as p r e v i o u s l y n o t e d a r e c a r r i e d o u t i n n i t r i c s u l p h u r i c m i x e s s i n c e t h i s a f f o r d s a cheap method f o r p r o d u c t i o n o f h i g h n i t r o g e n c o n t e n t m a t e r i a l w i t h c o n v e n i e n t r e c o v e r y and w i t h a lack o f c o r r o s i o n problems. The m a i n p o i n t o f i n t e r e s t i n t e c h n i c a l n i t r a t i o n i n m i x e d a c i d i s t h e q u e s t i o n o f t h e d e g r e e o f s u b s t i t u t i o n and how t h i s r e l a t e s to the a c i d mix c o m p o s i t i o n . Thus a l t h o u g h F i g u r e 26 shows a maximum d e g r e e o f s u b s t i t u t i o n o f 3 , i n p r a c t i c e t h i s i s n e v e r a t t a i n e d and t h e maximum d e g r e e o f s u b s t i t u t i o n i s % 2 . 8 . A d e f i n i t i v e answer as t o why t h i s s h o u l d be so h a s n o t t h u s f a r b e e n g i v e n s i n c e i n a heterogeneous p r o c e s s the d e l i n e a t i o n between the p o s s i b l e e x p l a n a t i o n s r e q u i r e s a t e c h n i q u e w h i c h c a n c l e a r l y d i s t i n g u i s h s u r f a c e from b u l k phenomena ( c f . F i g u r e 2 7 ) . The q u e s t i o n o f t h e l i m i t i n g d e g r e e o f s u b s t i t u t i o n ( u s u a l l y e s t a b l i s h e d by a b u l k n i t r o g e n d e t e r m i n a t i o n u s i n g a m i c r o K j e l d a h l technique) i n n i t r o c e l l u l o s e s could a p r i o r i be r a t i o n a l i z e d i n terms o f two e x t r e m e m o d e l s . The f i r s t c a n be a t t r i b u t e d t o t h e m i c r o and m a c r o s c o p i c s t r u c t u r e o f the c e l l u l o s e . Thus i n h o m o g e n e i t i e s i n t h e b u l k structure c o u l d c o n c e i v a b l y g i v e r i s e t o a c c e s s i b l e and i n accessible regions. S i n c e n i t r a t i o n must depend on t h e d i f f u s i o n of reagent throughout the b u l k s t r u c t u r e a f u r t h e r c o n s i d e r a t i o n i s t h a t t h e r e may w e l l be a c o n c e n t r a t i o n p r o f i l e t h r o u g h o u t t h e structure. On t h i s b a s i s t h e l e s s t h a n maximum d e g r e e o f s u b stitution is attributable t o an inhomogeneous b u l k structure c o r r e s p o n d i n g t o r e g i o n s o f c o m p l e t e l y n i t r a t e d m a t e r i a l and unreacted i n a c c e s s i b l e regions. A n a l t e r n a t i v e and somewhat more p l a u s i b l e a l t e r n a t i v e i s that since n i t r a t i o n i s a r e v e r s i b l e e s t e r i f i c a t i o n process then 3

2

4

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTON,

ELECTRON,

Uneven nitration

Rel. high nitrogen

High nitrogen content

degradation

content. Cheap, ease

with small amount of

gelatinisation of

of recovery and lack

degradation. Corrosion

product.

of corrosion problems.

and recovery

Figure 25.

PROBES

Nitric- Phosphoric

Nitric - Sulphuric

Nitric acid

A N D ION

problems

Nitration of cellulose: possible reaction pathways

Degree of Substitution

Nitrogen Content 14-4

27-2-9

12-6 - 13.4 Gun Cottons

2-5-26

11.8-12.4 Photographic films

10.6-124

2-25-2.6

Nitro lacquers

10.6-11-2 Celluloid

2-25-2-4

Figure 26.

1. Maximum (a)

Nitrocelluloses DOS as a function of application

Degree

of Substitution

Inhomogeneities (i) In bulk

is

^

2-8

why ?

?

structure

Iii) In nitrating mix (iii) Combination of (i) and (ii)

(b) Figure 27.

Kinetic and Thermodynamic

reasons.

Points of interest in the technical nitration in mixed acid of cellulose

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

17.

CLARK

ESCA

Studies of Polymer Surfaces

the the

273

^ 2 . 8 degree o f s u b s t i t u t i o n t y p i c a l l y observed r e p r e s e n t s equilibrium situation ( c f . Figure 28). I n o r d e r t o shed l i g h t on t h i s s i t u a t i o n we p r e s e n t h e r e a c o m p a r i s o n o f t h e s u r f a c e and b u l k n i t r a t i o n o f c e l l u l o s e i n t h e form o f l i n t e r s p a p e r (_17). B e f o r e d i s c u s s i n g t h e d a t a however i t i s worthwhile b r i e f l y c o n s i d e r i n g the background concerning the b u l k s t r u c t u r e o f c e l l u l o s e and n i t r o c e l l u l o s e . The s t r u c t u r e o f c e l l u l o s e o r n i t r o c e l l u l o s e may b e d i s c u s s ed on t h r e e d i s t i n c t l e v e l s ( 1 4 , 1 8 ) . (i) The s t r u c t u r e o f the macroscopic f i b r e generally about 20u i n d i a m e t e r . S t r u c t u r a l B o t a n y h a s shown t h a t t h e f i b r e s a r e c h a r a c t e r i z e d by a number o f d i s t i n c t l a y e r s t h e r e s u l t o f t h e g r o w t h mechanism w i t h i n the l i v i n g p l a n t . (ii) The n a t u r e o f t h e s e l a y e r s w h i c h i n t h e c a s e o f c e l l u l o s e appea b d f fibril withi f i b r i l s , rangin t h e so c a l l e m e t e r and t h o u g h t t o c o n s i s t o f c h a i n s o f D g l u c o s e m o l e c u l e s 12 x 8 u n i t s i n c r o s s s e c t i o n . (iii) The m o l e c u l a r l e v e l i n c l u d i n g t h e u n i t c e l l o f t h e c r y s t a l l i n e r e g i o n s , molecular weight d i s t r i b u t i o n etc. F o r b o t h c e l l u l o s e and n i t r o c e l l u l o s e a c o n s i d e r a b l e body o f e v i d e n c e h a s b e e n g e n e r a t e d on a s p e c t s ( i i ) and ( i i i ) h o w e v e r c o n s i d e r a b l e c o n t r o v e r s y surrounds the i n f o r m a t i o n p e r t a i n i n g t o (ii). L e w i s ( 1 8 ) has s u c c i n c t l y d e s c r i b e d t h e c u r r e n t s t a t e o f k n o w l e d g e i n w h i c h t h e t h r e e m o d e l s d e p i c t e d i n F i g u r e 29 have been d i s c u s s e d . Most o f t h e o l d e r l i t e r a t u r e d i s c u s s e s t h e c h e m i s t r y o f c e l l u l o s e i n terms o f t h e w e l l known f r i n g e d m i c e l l e m o d e l ( a ) . The m a i n f e a t u r e o f t h i s m o d e l i s an e x t e n d e d c r y s t a l l i n e s t r u c t u r e i n t e r s p e r s e d w i t h amorphous r e g i o n s . X - r a y s t u d i e s and more r e c e n t c h e m i c a l e v i d e n c e p r o v i d e l i t t l e support f o r such a model and an a l t e r n a t i v e , m i c r o f i b r i l m o d e l ( b ) h a s b e e n d e v e l o p e d . The f u n d a m e n t a l a s p e c t o f t h i s m o d e l i s t h a t v i r t u a l l y 100% o f t h e c o t t o n i s i n t h e c r y s t a l l i n e f o r m and c o n s i s t s o f u l t i m a t e f i b r i l s 3-5 u i n d i a m e t e r . The amorphous o r non c r y s t a l l i n e r e g i o n s i n t h i s model are a s s o c i a t e d w i t h the s u r f a c e r e g i o n s o f these microfibrils. The v a r y i n g d e g r e e o f a c c e s s i b i l i t y t o c h e m i c a l r e a g e n t s i s e x p l a i n e d by p o s t u l a t i n g t h a t t h e number o f a c t i v e c e n t e r s on any p a r t i c u l a r c e l l u l o s e m o l e c u l e w i l l depend on whether i t l i e s a l o n g the c o r n e r , edge, the face or the i n t e r i o r o f the m i c r o f i b r i l . The l a r g e r t h e f i b r i l d i a m e t e r t h e s m a l l e r t h e p r o p o r t i o n o f non c r y s t a l l i n e m a t e r i a l p r e s e n t . I n the case o f n i t r o c e l l u l o s e e x p e r i m e n t a l e v i d e n c e has been p r e s e n t e d which i s b a s i c a l l y i n c o m p a t i b l e w i t h s u c h a m o d e l . A compromise b e t w e e n t h e f r i n g e d m i c e l l e and t o t a l l y c r y s t a l l i n e m i c r o f i b r i l theory has r e c e n t l y b e e n s u g g e s t e d , t h e m a i n f e a t u r e o f t h e m o d e l ( c ) b e i n g t h a t amorphous r e g i o n s b e t w e e n c r y s t a l l i t e s r u n i n c o n t i n u -

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

P H O T O N , E L E C T R O N , A N D ION PROBES

274

Older Theory

In) Amorphous regions

Crystalline

Nitrating

Mix

as Nitration

proceeds

H 0|

HN0 | 3

2

Accessible and Inaccessible

regions. first formed nitrate.

Figure 28.

Inhomogeneities leading to reduced DOS for bulk materials

(b) Ki

0 - NO2 + HNO3. 2 ° H

Ii)

o-H

+ 2HNO3

HN0 .H 0 3

^

2

0-H

*

K = —

Competitive -~

(iii)

Work

H

2° 0

"

Nitration - denitration rapidly

N

2

0

+

H



equlibrium

established ?

sulphonation

0-H up

HNO3 *

HNO3

2

Iii)

rt=^

+ H S0^

^ 0 - S 0 3 H + H

2

2

0

procedure

Hydrolysis 0 - N0

Figure 29.

2

+ H 0 2

^r- 0 - H + HNO3

Kinetics and equilibria involved in nitrations in mixed acids

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

17.

CLARK

ESCA

Studies of Polymer Surfaces

275

ous v e i n s t h r o u g h t h e p o l y m e r and a r e n o t r a n d o m l y d i s t r i b u t e d as i n the f r i n g e d m i c e l l e t h e o r y . The a v a i l a b l e d a t a p a r t i c u l a r l y that r e l a t i n g to p r o c e s s i n g o f n i t r o c e l l u l o s e i s compatible w i t h s u c h a m o d e l w i t h t h e m i c r o f i b r i l s as i n ( b ) b e i n g t h e c r y s t a l l i t e s w i t h t h e i n t e r v e n i n g r e g i o n s b e i n g f i l l e d w i t h amorphous m a t e r i a l w h i c h d i f f e r s l i t t l e i n d e n s i t y from t h a t o f t h e c r y s t a l l i n e r e g i o n s ( s e e F i g u r e 30) . I n any h e t e r o g e n e o u s p r o c e s s i t i s t h e s u r f a c e w h i c h p r o v i d e s a " w i n d o w " on t h e r e a c t i o n however as we have p r e v i o u s l y n o t e d t h e s t u d i e s t o d a t e have f o c u s s e d on t h e b u l k c h e m i s t r y . M i l e s ( 1 4 ) has summarized much o f t h e i m p o r t a n t l i t e r a t u r e i n t h e p e r i o d t o c i r c a 1950 and e v i d e n c e h a s b e e n p r e s e n t e d t h a t u n d e r c e r t a i n c o n d i t i o n s e q u i l i b r i u m b e t w e e n n i t r a t i o n and d e n i t r a t i o n i s e s t a b l i s h e d on t h e t y p i c a l t i m e s c a l e i n v o l v e d f o r n i t r a t i o n . V e r y r e c e n t ( 1 3 ) C nmr s t u d i e s have c o n f i r m e d ( 1 9 ) an e a r l i e r c h e m i c a l l y based i n f e r e n c (20) tha i nitrated cellulos f g i v e n degree o f s u b s t i t u t i o r a t i o s (primary v s secondary ) i s important to recognize that t n i s almost c e r t a i n l y corresponds t o an e q u i l i b r i u m o r d e r o f r e a c t i v i t y s i n c e i t i s known t h a t i n homogeneous r e a c t i o n s t h e r a t e c o n s t a n t s f o r d e n i t r a t i o n o f p r i mary e s t e r s i s s u b s t a n t i a l l y s m a l l e r t h a n f o r n i t r a t i o n u n d e r a g i v e n s e t o f c o n d i t i o n s (2_1). The c h e m i c a l r e a c t i o n s e q u e n c e involving treatment o f the n i t r a t e d material with Nal i n a c e t o n y l a c e t o n e p r o v i d e s i n f o r m a t i o n on t h e t o t a l l e v e l o f p r i mary n i t r a t e e s t e r g r o u p s i n t h e sample ( 2 0 ) . By c o n t r a s t t h e h i g h r e s o l u t i o n (1_3) C nmr s t u d i e s r e l a t e o n l y t o t h e s o l u b l e fraction of nitrated material. The i n f o r m a t i o n d e r i v e d f r o m t h e s e c h e m i c a l and s p e c t r o s c o p i c s t u d i e s a r e v a l u a b l e b u t s t i l l l e a v e i m p o r t a n t gaps i n t h e i n f o r a m t i o n r e q u i r e d t o f u l l y u n d e r s t a n d t h e n i t r a t i o n o f c e l l u l o s e i n m i x e d a c i d s . I n o r d e r t o shed new l i g h t on t h i s t e c h n o l o g i c a l l y and a c a d e m i c a l l y i m p o r t a n t a r e a we p r e s e n t h e r e a d e t a i l e d c o m p a r i s o n o f t h e s u r f a c e and b u l k n i t r a t i o n and d e n i t r a t i o n o f c e l l u l o s e m a t e r i a l s i n terms o f d e g r e e o f s u b s t i t u t i o n and r e l a t i v e r a t e s o f r e a c t i o n s . ?

Experimental. S i n c e the n i t r a t i o n o f c e l l u l o s e i n the form o f l i n t e r s p a p e r may w e l l b e u n f a m i l i a r t o many p e o p l e we p r e s e n t here a b r i e f synopsis o f the procedures i n v o l v e d . The g e n e r a l procedures are i n d i c a t e d schematically i n Figure 31. Commerciall y p r o d u c e d l i n t e r s p a p e r ( H e r c u l e s Powder C o . ) o f S h i r l e y f l u i d i t y 8 . 8 and a p p r o x i m a t e d e g r e e o f p o l y m e r i z a t i o n 1100 was vacuum d r i e d i n an oven a t 60 f o r 2 h r s . and s t o r e d o v e r P « 0 , - . This p r o v i d e s a s t a r t i n g c e l l u l o s e sample w i t h < 2% w a t e r . Nitrations and d e n i t r a t i o n s were a c c o m p l i s h e d by i m m e r s i o n i n t h e a p p r o p r i a t e a c i d m i x ( o f sample s t r i p s ( 4 cm x 4 cm) f o r a g i v e n p e r i o d o f t i m e u s i n g an a p p a r a t u s s u c h as t h a t d e p i c t e d i n F i g u r e 3 2 . B u l k n i t r o g e n d e t e r m i n a t i o n s have b e e n c a r r i e d o u t u s i n g a m i c r o Kjeldahl.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Figure 30. Possible models for the structure of cellulose and nitrocellulose (after Ref. 18;

c)

THE VEINED MODEL

Cellulose paper type 1 Ex M.O.D. dried at 60° in vacuum oven. 2hrs.

I Stored over P2O5 4x4cm. strips immersed in Nitrating acid by trome or Mitchell method.

As for Nitration always Mitchell method.

Atter specfic time, strips are removed from mix, squeezed free of acid on a Buchner funnel and quenched in 1 litre of water of 5°C.

Nitrocellulose is washed in 3» 500 mis. of cold water and further boiled for 2-3 hours.

Nitrocellulose (high N content) placed in spent acrd of mix used to Nitrate Cellulose to a low D.O.S.

Soxhlet washing hos also been tried.

Work up as for Nitrotion

Drying proceedure as above. ESCA and Kjeldahl analysis carried out.

Figure 31. Nitration and denitration procedures (nitrations according to Mitchell procedure) (22)

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

17.

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Preliminary Analysis. The c o r e l e v e l ( C i s , O l s , N i s ) s p e c tra for a typical nitrocellulose sample which has been c o m m e r c i a l l y p r o d u c e d i s shown i n F i g u r e 3 3 . The C i s s p e c t r u m shows 3 d i s t i n c t i v e components ( o n c e a s t a n d a r d l i n e shape a n a l y s i s h a s b e e n c a r r i e d o u t ) . Thus t h e c e n t r a l component a t ^287.4 eV a r i s e s p r e d o m i n a n t l y from t h e c a r b o n s C 2 , C3 and C6 b e a r i n g t h e n i t r a t e e s t e r f u n c t i o n a l i t y w i t h c o n t r i b u t i o n s from C4 and C 5 . C I u n i q u e l y a t t a c h e d t o two oxygens i n t h e c y c l i c h e m i a c e t a l f o r m u l a t i o n o f the 3-Dglucopyranose r i n g i s at h i g h e s t b i n d i n g energy. The component a t 285 eV a r i s e s f r o m e x t r a n e o u s h y d r o c a r b o n w h i c h we w i l l show l a t e r i s c o n f i n e d t o t h e v e r y s u r f a c e o f t h e s a m p l e . I n c e l l u l o s e samples i r r e s p e c t i v e o f source o r t y p e ( e . g . l i n t e r s or s t a p l e f i b r e s ) t h e h y d r o c a r b o n s i g n a l o b s e r v e d i s v a r i a b l e b u t inevitably present. The N i s s i g n a l c o n s i s t s o f an i n t e n s e h i g h b i n d i n g e n e r g y component and ( c f . F i g . 3) c o m p a r i s o n w i t h m o d e l s y s t e m s u n a m b i g u o u s l y i d e n t i f i e s t h i s as o r i g i n a t i n g f r o m t h e n i t r a t e e s t e r groups -O-NO^ b i n d i n g e n e r g y componen associated with n i t r i t e ester groups. I n the l a b o r a t o r y s c a l e n i t r a t i o n s where c o n d i t i o n s c a n p e r h a p s be more p r e c i s e l y c o n t r o l l e d such s t r u c t u r a l f e a t u r e s a r e a t a much l o w e r l e v e l . Conventional n i t r i t e traps (urea, ascorbic acid e t c . ) dissolved i n the n i t r a t i n g m i x to o b v i a t e the p o s s i b i l i t y o f f o r m a t i o n o f n i t r i t e e s t e r s does n o t seem t o remove t h e v e r y l o w l e v e l s o f s u c h s t r u c t u r a l f e a t u r e s and i t c o u l d be t h a t t h e y a r i s e as a r e s u l t o f r e a c t i o n s o c c u r r i n g d u r i n g s t a b i l i z a t i o n and s t o r a g e . The O l s l e v e l s a r e an u n r e s o l v e d b r o a d p e a k . With a knowledge o f s e n s i t i v i t y f a c t o r s f o r the v a r i o u s core l e v e l s i t i s p o s s i b l e t o s t r a i g h t f o r w a r d l y work o u t t h e d e g r e e o f s u b s t i t u t i o n DOS ( a v e r a g e number o f n i t r o - e s t e r functionalities per glucose r e s i d u e ) . Thus t h e i n t e g r a t e d C l s / N l s a r e a r a t i o s ( e x c l u d i n g t h e e x t r a n e o u s h y d r o c a r b o n component) y i e l d s a DOS o f 2.3 f o r the p h o s p h o r i c / n i t r i c n i t r a t i n g m i x i d e n t i c a l w i t h t h a t d e t e r m i n e d from m i c r o K j e l d a h l b u l k a n a l y s i s . The t o t a l C l s / O l s r a t i o i n d i c a t e s that there i s l i t t l e r e s i d u a l water i n the n i t r a t e d sample. W i t h f i b r i l l a r samples s u c h as l i n t e r p a p e r s any i n f o r m a t i o n on v e r t i c a l i n h o m o g e n e i t i e s i n t o t h e sample may o n l y be i n f e r r e d by l o o k i n g a t d i f f e r e n t l e v e l s c o r r e s p o n d i n g to d i f f e r e n t e s c a p e depths. C l e a r l y t h e good agreement b e t w e e n b u l k and s u r f a c e a n a l y s e s s u g g e s t s t h a t t h e sample i s u n i f o r m a l l y n i t r a t e d and t h e e x t r a n e o u s h y d r o c a r b o n must t h e r e f o r e be l o c a l i z e d a t t h e s u r f a c e p e r h a p s i n t h e form o f a p a t c h e d o v e r l a y e r s i n c e i n most c a s e s t h e r e may w e l l be s i g n i f i c a n t l y l e s s t h a n a m o n o l a y e r p r e s e n t . One way o f e s t a b l i s h i n g t h e p u r e l y s u r f a c e n a t u r e o f t h i s h y d r o c a r b o n ( s i n c e the u s u a l a n g u l a r dependent s t u d i e s a r e n o t f e a s i i b l e ) i s t o compare DOS a v e r a g e d o v e r d i f f e r e n t s a m p l i n g d e p t h s . Whereas f o r M g K t h e t y p i c a l s a m p l i n g d e p t h w i l l b e s a y ^ 5 0 A f o r T i K w i t h p h o t o n e n e r g y 4510 eV a f i g u r e o f ^ 3 0 0 S w o u l d b e more appropriate. a

a

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

278

P H O T O N , E L E C T R O N , A N D ION PROBES

CLAMP

SS SAMPLE HOLDER

STIRRER

CONSTANT TEMP. BATH

Figure 32.

Nitrating apparatus used in ESCA studies

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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F i g u r e 34 shows N i s and C i s s p e c t r a f o r c e l l u l o s e l i n t e r s p a p e r n i t r a t e d f o r 300 s e e s , i n a l o w n i t r a t i n g m i x . Whereas t h e M g K s p e c t r a f o r t h e C i s l e v e l s show a s i g n i f i c a n t c o n t r i b u t i o n from t h e e x t r a n e o u s hydrocarbon the c o r r e s p o n d i n g lineshape a n a l y s i s f o r t h e T i K s p e c t r a i s d e s c r i b e d w i t h no c o n t r i b u t i o n from h y d r o c a r b o n s i n c e even monolayer coverage 5$) w o u l d c o n t r i b u t e a n e g l i g i b l e c o n t r i b u t i o n to the C i s l e v e l s f o r e l e c t r o n s h a v i n g a mean f r e e p a t h a t l e a s t an o r d e r o f m a g n i t u d e greater 9 0 £ ) . T h i s shows t h e g r e a t v a l u e o f h a v i n g a v a r i a b l e photon source r o u t i n e l y a v a i l a b l e . The r e l a t i v e s e n s i t i v i t y o f n i t r o c e l l u l o s e s t o p h o t o c h e m i c a l d e c o m p o s i t i o n s i s known and i t i s t h e r e f o r e n e c e s s a r y t o i n v e s t i g a t e t h e s e n s i t i v i t y t o i n t e r r o g a t i o n by means o f ESCA o f n i t r a t e d and d e n i t r a t e d c e l l u l o s e s a m p l e s . I t may r e a d i l y b e shown t h a t on t h e t y p i c a l t i m e s c a l e f o r t h e ESCA i n v e s t i g a t i o n s photochemical degradation i s n e g l i g i b l e A t t h e t y p i c a l dose rates involved ( t y p i c a signs o f decomposition The m a i n r e a c t i o n a p p e a r s t o be p h o t o - r e d u c t i o n . Thus t h e h i g h b i n d i n g e n e r g y component i n t h e N i s s p e c t r u m a p p r o p r i a t e t o -O-NO^ s t r u c t u r a l f e a t u r e s i s a c c o m p a n i e d i n t h e c a s e o f m a t e r i a l s u b j e c t e d t o i r r a d i a t i o n f o r e x t e n d e d p e r i o d s by a s m a l l peak a t low b i n d i n g e n e r g y a t t r i b u t a b l e t o - C - N H f u n c t i o n a l i t y . Even after 5 h o u r s i r r a d i a t i o n however t h i s component s t i l l only r e p r e s e n t s a s m a l l f r a c t i o n o f the t o t a l N i s spectrum. We may c o n c l u d e from t h i s t h a t X - r a y d e g r a d a t i o n i s n e g l i g i b l e d u r i n g t h e t i m e s c a l e o f a t y p i c a l ESCA i n v e s t i g a t i o n . a

a

Detailed S t u d i e s O f N i t r a t i o n And D e n i t r a t i o n . For a n i t r a t i n g m i x o f a g i v e n c o m p o s i t i o n one o f t h e most i m p o r t a n t f e a t u r e s o f i n t e r e s t i s t h e q u e s t i o n as t o how r a p i d l y an e q u i l i b r i u m DOS i s e s t a b l i s h e d i n t h e s u r f a c e r e g i o n s a c c e s s i b l e t o ESCA. A s an e x a m p l e F i g u r e 35 shows c o r e l e v e l s p e c t r a f o r samples o f t h e same b a t c h o f l i n t e r s p a p e r s n i t r a t e d f o r d i f f e r i n g p e r i o d s i n a n i t r a t i n g m i x c o n s i s t i n g o f 75% H ^ S O ^ : 22.2% HNO^: 2.8% H ^ O . The N i s l e v e l s , w h i c h p r o v i d e a r e a d y means o f f o l l o w i n g t h e n i t r a t i o n , r e m a i n c o n s t a n t i n i n t e n s i t y from r e a c t i o n times o f 1 sec. to 1 hour. T h i s i n d i c a t e s t h a t on t h e ESCA d e p t h scale, equilibrium i s very r a p i d l y established. This i s not e n t i r e l y unexpected s i n c e the d i f f u s i o n o f n i t r a t i n g mix i n t o the o u t e r m o s t 50& o r so o f t h e c e l l u l o s e f i b r i l s i s e x p e c t e d t o o c c u r rapidly. A s i m i l a r c o m p a r i s o n i s shown i n F i g u r e 36 f o r s a m p l e s s t u d i e d by means o f t h e h a r d e r T i K X-ray source. Here the s a m p l i n g d e p t h w i l l be s e v e r a l h u n d r e d A n g s t r o m s y e t t h e s p e c t r a r e c o r d e d a f t e r r e a c t i o n t i m e s o f 1 s e c . and 300 s e c . a r e c l o s e l y similar. It is clear therefore t h a t t h e e q u i l i b r i u m DOS i s r a p i d l y e s t a b l i s h e d i n the surface r e g i o n s . a

The g r o s s f e a t u r e s o f t h e e q u i l i b r i a w h i c h a r e i n v o l v e d i n d e t e r m i n i n g t h e o v e r a l l DOS i n t h e s u r f a c e r e g i o n s o f c o t t o n f i b r i l s i s o u t l i n e d s c h e m a t i c a l l y i n F i g u r e 3 7 . F o r mixed a c i d

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

280

P H O T O N , E L E C T R O N , A N D ION

Figure 34.

PROBES

The N-ls and C-ls spectra for nitrocellulose produced in mixed acid (MgKai, and TiK spectra) 2

ai>2

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Figure 35.

ESCA

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281

Core-level spectra as a function of time for nitration in mixed acid

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

282

P H O T O N , E L E C T R O N , A N D ION

416

412

408 404

293

PROBES

289 285

Figure 36. The TiK spectra for nitration of cellulose for 1 s and 300 s in mixed acid a

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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CLARK

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n i t r a t i n g m i x e s i t h a s been p r o p o s e d t h a t t h e s u l p h u r i c a c i d component does n o t d i f f u s e i n t o t h e b u l k ( 1 4 ) and t h i s c o u l d o b v i o u s l y l e a d t o d i f f e r e n c e s i n DOS f o r t h e s u r f a c e and b u l k r e g i o n s o f samples s i n c e o n l y i n the s u r f a c e r e g i o n s i s s u l p h o n a tion competitive with n i t r a t i o n . An important q u e s t i o n which ESCA i s p o t e n t i a l l y c a p a b l e o f a n s w e r i n g i s w h e t h e r sulphate e s t e r s may be d e t e c t e d i n t h e s u r f a c e r e g i o n s o f n i t r a t e d m a t e rial. M g K ^ £ s p e c t r a f o r t h e N i s , S2p and C i s r e g i o n s o f c o t t o n l i n t e r s for* n i t r a t e d m a t e r i a l a r e shown i n F i g u r e 3 8 . A l o w l e v e l S2p s i g n a l i s d e t e c t e d and t h e h i g h b i n d i n g e n e r g y o f ^ 169 eV i d e n t i f i e s t h i s as a r i s i n g from s u l p h a t e e s t e r g r o u p s . It is clear therefore that sulphate e s t e r s a r e formed i n t h e v e r y surface regions. Comparable s t u d i e s w i t h t h e T i K X - r a y source w i t h a l a r g e r s a m p l i n g d e p t h shows v i r t u a l l y no e v i d e n c e f o r s u l p h a t e e s t e r s and ESCA t h e r e f o r e uniquel demonstrates the s u r f a c e n a t u r e o f such The f i n a l DOS f o r s u r f a c e r e g i o n s i s c o n c e r n e d , depends on n i t r a t i o n , d e n i t r a t i o n , sulphonation e q u i l i b r i a . The f a c t that t h e DOS i s r a p i d l y e s t a b l i s h e d suggests that d e n i t r a t i o n i s competitive w i t h n i t r a t i o n w h i l s t t h e f a c t t h a t e v e n i n h i g h s u l p h u r i c m i x e s t h e DOS i s s t i l l appreciable i l l u s t r a t e s that n i t r a t e ester formation i s more f a c i l e t h a n s u l p h a t e e s t e r f o r m a t i o n . The r a p i d i t y w i t h which d e n i t r a t i o n - n i t r a t i o n e q u i l i b r i a are e s t a b l i s h e d i n the s u r f a c e r e g i o n s i s n i c e l y i l l u s t r a t e d by t h e d a t a d i s p l a y e d i n F i g u r e 3 9 . Thus f o r s t a r t i n g m a t e r i a l , DOS 2 . 7 d e n i t r a t i o n o f t h e o u t e r m o s t few t e n s o f a n g s t r o m s i s r a p i d and d e p e n d e n t on a c i d m i x . I n 79.1% HNO^ d e n i t r a t i o n i n 1 s e c . i s t o DOS 2 . 3 s i n c e t h i s m i x i s c l o s e t o t h e minimum n e c e s s a r y f o r n i t r a t i o n t o be e f f e c t e d (77.8% H N 0 c o r r e s p o n d s t o t h e m o n o h y d r a t e ) w h i l s t f o r 84.4% H N 0 d e n i t r a t i o n i s t o a DOS o f 2 . 5 on a 1 s e c . t i m e s c a l e . Corresponding s p e c t r a w i t h a T i K X - r a y source i n d i c a t e s t h a t the degree o f d e n i t r a t i o n i s s l i g h t l y lower than for the M g K X - r a y source. The e x t r e m e s e n s i t i v i t y o f ESCA i n t h e d e t e c t i o n o f t h e i n i t i a l stages o f r e a c t i o n s i s n i c e l y d i s p l a y e d by t h e comp a r a t i v e d a t a g i v e n i n F i g u r e 4 0 . T h i s shows t h e n i t r a t i o n and d e n i t r a t i o n o f c e l l u l o s i c samples. The s u b s t a n t i a l secondary s h i f t o f t h e n i t r a t e e s t e r group i s shown by t h e s h i f t t o h i g h b i n d i n g e n e r g y compared w i t h t h e e x t r a n e o u s h y d r o c a r b o n peak (AE 2 . 3 eV f o r t h e C-O-NO^ component compared w i t h AE 1.7 eV f o r cellulose itself. The ESCA d a t a p r e s e n t e d , thusfar therefore establishes that n i t r a t i o n - d e n i t r a t i o n - s u l p h o n a t i o n e q u i l i b r i a are r a p i d l y e s t a b l i s h e d i n the surface r e g i o n s . S y s t e m a t i c s t u d i e s have been made o f n i t r a t i o n s i n b o t h n i t r i c - p h o s p h o r i c and n i t r i c - s u l p h u r i c m i x e s , b o t h from a s u r f a c e and b u l k p o i n t o f v i e w . F o r n i t r i c p h o s p h o r i c m i x e s t h e DOS depends on t h e c o m p o s i t i o n and t h e r e i s l i t t l e e v i d e n c e f o r formation o f phosphate e s t e r s . The DOS i n t h e s u r f a c e r e g i o n t h e r e f o r e r e p r e s e n t s t h e e q u i l i b r i u m b e t w e e n n i t r a t i o n and d e a

3

3

a

a

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Figure 38. The N-ls, C-ls, and S-2p core-level spectra for nitrated material (detection of sulfate esters)

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

17.

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Figure 39.

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Denitration of nitrocellulose DOS 2.7 as a function of denitrating mix (MgKaij)

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

286

P H O T O N , E L E C T R O N , A N D ION

Figure 40.

PROBES

Comparison of surface nitration and denitration of cellulose

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Figure 41.

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30

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In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

L5

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Figure 42.

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289

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Binding Energy (eV)

Figure 8.

The C-ls and F-ls binding energies measured for the linear luoropolymers

F, (x004) .... s

' 692 ' 690 ' 688

Figure 9.

—I

1

r—

686 296 294 Binding energy (eV)

292

290 288

286

284

Core-level spectra of plasma polytetrafluoroethylene

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

18.

DILKS

XPS Studies of Polymer Surfaces

305

large. A s i m p l e a d d i t i v e m o d e l may be c o n s t r u c t e d t o d e s c r i b e t h e p r i m a r y s u b s t i t u e n t e f f e c t o f f l u o r i n e on c a r b o n , where t h e s h i f t i n b i n d i n g energy o f a g i v e n C i s core l e v e l e l e c t r o n subsequent t o r e p l a c i n g h y d r o g e n o r c a r b o n by f l u o r i n e i s ^ 2 . 7 e V p e r f l u o r i n e substituent. I t i s a l s o a p p a r e n t from F i g u r e 8 t h a t s u b s t i t u t i o n o f f l u o r i n e on t h e n e x t c a r b o n atom a l o n g t h e c h a i n a l s o p r o d u c e s a s i g n i f i c a n t , s e c o n d a r y s h i f t , w h i c h h a s b e e n shown t o be ^ 0 . 4 e V per beta f l u o r i n e s u b s t i t u e n t (8). While the C i s spectra o f s i m p l e l i n e a r f l u o r o p o l y m e r s a r e w e l l r e s o l v e d due t o t h e l a r g e p r i m a r y s h i f t s , t h e c o r r e s p o n d i n g s p e c t r a f o r more c o m p l i c a t e d f l u o r o p o l y m e r s t r u c t u r e s ( e . g . as found i n p l a s m a p o l y m e r i z a t i o n ) t e n d t o be r a t h e r u n r e s o l v e d due t o t h e r e l a t i v e l y l a r g e r a n g e o f s e c o n d a r y s h i f t e f f e c t s on a g i v e n s t r u c t u r a l f e a t u r e w h i c h may o c c u r i n a v a r i e t y o f c h e m i c a l l y d i f f e r e n t e n v i r o n m e n t s (9^). ( F o r e x a m p l e , a - ^ C F s t r u c t u r a l f e a t u r e bonded t o t h r e e / C F ^ g r o u p s w i l l have a C i s b i n d i n g e n e r g y ^ 2eV h i g h e r t h a n f o r a ^ C F s t r u c t u r a l feature havin F i g u r e 9 shows t h ized tetrafluoroethylene film produced in a radiofrequency c a p a c i t i v e l y coupled diode reactor c o n f i g u r a t i o n ( 9 ) . The C i s spectrum c l e a r l y e x h i b i t s several distinct types o f carbon e n v i r o n m e n t and c o m p a r i s o n w i t h p r e v i o u s l y w e l l characterized systems a l l o w s t h e i n d i v i d u a l components t o be a s s i g n e d to particular structural features. Thus t h e peak c e n t e r e d a t ^ 2 9 4 . l e V on t h e b i n d i n g e n e r g y s c a l e i s a t t r i b u t a b l e to carbon a t t a c h e d t o t h r e e f l u o r i n e atoms i n CF~ w h i l e t h o s e a t ^ 2 9 1 . 9 e V and 2 8 9 . 7 e V a r e due t o C¥^ and CF s t r u c t u r a l f e a t u r e s r e s p e c t i v e ly. The s i g n a l c e n t e r e d a t ^ 2 8 7 . 6 e V c a n be a s s i g n e d t o c a r b o n atoms n o t d i r e c t l y a t t a c h e d to f l u o r i n e but h a v i n g f l u o r i n e s u b s t i t u e n t s i n a b e t a p o s i t i o n , w h i l e t h e component a t ^ 2 8 5 . 0 e V a r i s e s from e i t h e r c a r b o n atoms i n a h i g h l y c r o s s l i n k e d e n v i r o n m e n t , h a v i n g no a l p h a o r b e t a f l u o r i n e s u b s t i t u e n t s , o r from a s m a l l amount o f h y d r o c a r b o n c o n t a m i n a t i o n . The f u l l w i d t h a t h a l f maximum (FWHM) o f each component i s r e l a t i v e l y l a r g e , i n d i c a t i n g a v a r i e t y o f environments f o r each s t r u c t u r a l t y p e . S i m i l a r data f o r plasma p o l y t r i f l u o r o e t h y l e n e , plasma p o l y 1,1 d i f l u o r o e t h y l e n e and p l a s m a p o l y f l u o r o e t h y l e n e a r e p r e s e n t e d i n F i g u r e 10 ( 9 ) . The components o f t h e C i s s p e c t r a o f t h e s e r i e s of fluorinated ethylenes are assigned as i n T a b l e 1, where s u b s c r i p t s a and b r e f e r t o t h e s t r u c t u r a l t y p e b e i n g i n a l a r g e l y f l u o r i n a t e d or l a r g e l y hydrocarbon environment r e s p e c t i v e l y . A f u l l d e s c r i p t i o n o f t h e l i n e s h a p e a n a l y s i s c a n b e found e l s e w h e r e (1, 9). The XPS d a t a r e v e a l t h a t a c o n s i d e r a b l e d e g r e e o f r e a r r a n g e ment o f t h e i n j e c t e d f l u o r o c a r b o n i s i n v o l v e d i n the plasma polymerization process. The r e l a t i v e q u a n t i t i e s o f C F ^ , C F ^ , CF and n o n - f l u o r i n e s u b s t i t u t e d f e a t u r e s a r e r e a d i l y m o n i t o r e d by XPS and show a s t r o n g dependence on t h e i n j e c t e d f l u o r o c a r b o n . A s t h e f l u o r i n e c o n t e n t o f t h e i n j e c t e d m a t e r i a l d e c r e a s e s so does t h a t o f the p o l y m e r . T a b l e 2 l i s t s t h e measured F / C s t o i c h i o -

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

306

PHOTON, ELECTRON,

Table

A N D ION PROBES

I

C i s and F l s peak a s s i g n m e n t s f o r t h e p l a s m a p o l y m e r s d e r i v e d e t h y l e n e and t h e s e r i e s o f f l u o r i n a t e d e t h y l e n e s .

CF3

Material

CH CH 2

CF

2

CF

Binding Energies (eV) c b a a

CH

C F

from

F

285.0

2

CH CHF

290.0

2

287.4

285. .9

285.0

687.3

285. .8

285.0

687.6

cis CHFCHF

292.9

290.0

287.7

287 .0

trans CHFCHF CF CH

292.7

290.0

287.8

286 .8

285. .8

285.0

687.5

2

293.4

290.6

288.2

287 .0

286. .0

285.0

688.4

CF CHF

294.0

291.

CF CF

294.1

291.9

285.0

689.5

2

2

2

2

287 .6

289.7

Table

II

F l u o r i n e / C a r b o n s t o i c h i o m e t r i e s d e r i v e d from t h e XPS d a t a f o r s e r i e s o f plasma polymers s t u d i e d .

Injected Material CH CH

Stoichiometry (F/C) From F l s / C l s From C i s comps. 2

0.00

0.00

CH CHF

0.14

0.26

CHFCHF

0.40

0.52

t r a n s CHFCHF

0.40

0.51

2

0.51

0.61

CF CHF

0.99

0.99

CF CF

1.33

1.33

2

2

cis

CF CH 2

2

2

2

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

the

18.

DILKS

XPS Studies of Polymer Surfaces

307

m e t r i e s o f t h e f i l m s d e r i v e d from t h e s e r i e s o f e t h y l e n e and t h e fluorinated ethylenes. The f i r s t c o l u m n i n d i c a t e s t h e i n j e c t e d m a t e r i a l . The s e c o n d i s t h e F / C s t o i c h i o m e t r y d e t e r m i n e d f r o m t h e F l s / C l s t o t a l s i g n a l i n t e n s i t y r a t i o c o r r e c t e d f o r the r e l a t i v e s e n s i t i v i t y o f these core l e v e l s , w h i l e the f i n a l column p r o v i d e s the F / C s t o i c h i o m e t r y determined from the r e l a t i v e i n t e n s i t i e s o f the C i s components. F o r plasma p o l y t e t r a f l u o r o e t h y l e n e and p l a s m a p o l y t r i f l u o r o e t h y l e n e t h e two v a l u e s agree p r e c i s e l y . However, i t i s c l e a r t h a t t h i s i s not the case f o r the o t h e r p l a s m a f l u o r o p o l y m e r s , f o r w h i c h t h e d e r i v a t i o n from t h e C i s l e v e l s always g i v e s a h i g h e r f i g u r e . T h i s c a n be a t t r i b u t e d t o t h e p r e s e n c e o f v i n y l i c CH w h i c h i s a c c o m p a n i e d by a s m a l l s h a k e up s a t e l l i t e p o s i t i o n e d ' W e V h i g h e r on t h e b i n d i n g e n e r g y s c a l e . T h i s adds a few p e r c e n t t o t h e measured i n t e n s i t i e s f o r t h e C F ^ and C F ^ components and a h i g h F / C s t o i c h i o m e t r y i s c a l c u l a t e d . The g r e a t e r t h e d i s c r e p a n c y b e t w e e n t h e two columns i n T a b l e 2 the g r e a t e r i s the v i n y l i along with supporting dat of t h e low m o l e c u l a r w e i g h t , n e u t r a l species i n the plasma effluents, have l e d t o t h e i d e n t i f i c a t i o n o f t h e most likely p r e c u r s o r to plasma p o l y m e r i z a t i o n i n these systems (9). I n an i n v e s t i g a t i o n o f m e t a l c o n t a i n i n g p l a s m a p o l y m e r s XPS has b e e n u s e d t o d e t e r m i n e b o t h t h e c a r b o n - f l u o r i n e s t r u c t u r e o f t h e p o l y m e r m a t r i x and t h e q u a n t i t y and f o r m i n w h i c h t h e m e t a l i s p r e s e n t ( 1 £ , 11). F i g u r e 11 shows t h e c o r e l e v e l s p e c t r a o f polymers produced at the grounded e l e c t r o d e i n a r a d i o f r e q u e n c y c a p a c i t i v e l y coupled diode r e a c t o r system using perfluoropropane as t h e i n j e c t e d gas ( K ) ) . The d a t a p e r t a i n t o t h r e e s e p a r a t e e x p e r i m e n t s i n w h i c h g e r m a n i u m , molybdenum and c o p p e r were u s e d as t h e e x c i t a t i o n e l e c t r o d e m a t e r i a l . The XPS d a t a i m m e d i a t e l y a f f i r m t h a t b o t h molybdenum and c o p p e r a r e i n c o r p o r a t e d i n t o t h e p o l y m e r f i l m s w h e r e a s germanium i s n o t u n d e r t h e c o n d i t i o n s u s e d . A n a l y s i s o f the r e l a t i v e s i g n a l i n t e n s i t i e s a l l o w the metal c o n t e n t s t o be e s t i m a t e d as ^20% and ^ 14% by w e i g h t f o r Mo and Cu respectively. I n t h e F l s s p e c t r a , i n a d d i t i o n t o t h e i n t e n s e peak at ^ 689.2eV a s s o c i a t e d w i t h f l u o r i n e attached to carbon there i s a s m a l l e r component a t l o w e r b i n d i n g e n e r g y ( ^ 6 8 5 . 5 e V ) , f o r t h e M o - C ^ F g and C u - C ^ F g s y s t e m s , due t o m e t a l f l u o r i d e . This, however, i s i n s u f f i c i e n t to account f o r the h i g h o x i d a t i o n s t a t e s o f t h e m e t a l s o b s e r v e d i n t h e Mo,j and C u ^ s p e c t r a . I n f a c t , as i s a p p a r e n t from t h e O l s r e g i o n s , "'She m e t a l s a r e p r e s e n t p r e d o m i n a t e l y as o x i d e s and h y d r o x i d e s w h i c h a r e formed on a i r e x p o s u r e (10). The s i m i l a r i t y o f t h e C i s s p e c t r a f o r t h e t h r e e f i l m s i n F i g u r e 11 s u g g e s t s t h a t t h e p o l y m e r s t r u c t u r e i s t h e same i r r e s p e c t i v e o f the e x c i t a t i o n e l e c t r o d e m a t e r i a l used. I t has been n o t e d t h a t t h e C i s s p e c t r a i n F i g u r e 11 a r e a l s o s i m i l a r t o t h a t o f p l a s m a p o l y t e t r a f l u o r o e t h y l e n e formed u n d e r s i m i l a r c o n d i t i o n s (9^, 12). This observation, c o u p l e d w i t h mass spectrometric a n a l y s i s o f the low m o l e c u l a r w e i g h t n e u t r a l s p e c i e s i n the plasma

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTON,

ELECTRON,

Plasma-polytrif luoroethylene

A N D ION PROBES

\

(c) \

(xOI)

/

• \ \

V

A/ V \ ."' _ \ '~ ~ x

v

Plasma-poly 1,1 difluoroethylene

(x0 25)

/?

'«/ x

^

"" "*

V.'/ \ \

-''

\ Plasma-polytiuoroethylene

^.y

--

692

690

688

686

684

294

292

290

288

286

284

282

Binding energy (eV)

Figure 10.

694

Core-level spectra of the plasma polymers derived from trifluoroethylene, 1,1 -difluoroethylene, and fluoroethylene

690

686

538

534

530 296

292

288

284

130

126

122

Binding Energy (eV)

Figure 11.

Core-level spectra of metal containing plasma fluoropolymers

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

18.

DILKS

309

XPS Studies of Polymer Surfaces

e f f u e n t s s u g g e s t e d a l i k e l y p o l y m e r i z a t i o n mechanism f o r t h e s e s y s t e m s i n v o l v i n g d i f l u o r o c a r b e n e and t e t r a f l u o r o e t h y l e n e as t h e primary precursors (10). S u r f a c e O x i d a t i o n o f P o l y m e r s . The i n v e s t i g a t i o n o f o x y g e n c o n t a i n i n g p o l y m e r s and more p a r t i c u l a r l y s u r f a c e o x i d i z e d p o l y m e r s , by XPS h a s b e e n g r e a t l y a i d e d by s e v e r a l r e c e n t p u b l i c a t i o n s p r o v i d i n g t h e n e c e s s a r y e x p e r i m e n t a l (1_, 2 6 , _13) and t h e o r e t i c a l (_13, ]A_) r e f e r e n c e d a t a . T h i s has been seen t o be o f g r e a t importance s i n c e , f o r the m a j o r i t y o f c a s e s , m o d i f i c a t i o n o f a polymer i n v o l v e s the formation o f oxygenated s t r u c t u r a l features at the s u r f a c e . I n c o n t r a s t t o t h e s i t u a t i o n found f o r f l u o r i n e as a s u b s t i t u e n t , t h e s e c o n d a r y s h i f t e x e r t e d by o x y g e n i s s m a l l (< 0 . 2 e V ) . T h i s a l l o w s us t o summarize s h i f t s i n C i s l e v e l s f o r o x y g e n - c o n t a i n i n g s t r u c t u r a l f e a t u r e s , i n terms o f t h e number o f c a r b o n - o x y g e n b o n d s , as f o l l o w s (6^ 1 5 ) : y

/° 0

=

£ \

0 ^290.6eV

o=c-o

>

^289.OeV

C=0

*

O-C-0

>

C-0

^287.9eV

^286.6eV

The r e f e r e n c e b i n d i n g e n e r g y f o r c a r b o n n o t a t t a c h e d t o o x y g e n ( e . g . i n p o l y e t h y l e n e ) i s 285.OeV a n d , as found i n t h e c a s e o f f l u o r i n e as a s u b s t i t u e n t , t h e p r i m a r y s u b s t i t u e n t e f f e c t o f o x y g e n c a n be d e s c r i b e d i n terms o f a s i m p l e a d d i t i v e m o d e l . The s h i f t i n b i n d i n g energy, o f a C i s core l e v e l e l e c t r o n , subsequent t o r e p l a c i n g c a r b o n o r h y d r o g e n by o x y g e n i s ^ 1 . 5 e V p e r c a r b o n oxygen bond. T h i s i s o n l y h a l f t h e s h i f t i n d u c e d by f l u o r i n e , and c o u p l e d w i t h t h e t y p i c a l f u l l w i d t h a t h a l f maximum (FWHM) o f a Cis level of ^ 1 . 4 e V w i t h the r e s o l u t i o n o f the spectrometer employed h e r e , m a n i f e s t s i t s e l f i n b r o a d u n r e s o l v e d C i s p r o f i l e s for o x i d i z e d polymers. Because the secondary s h i f t o f oxygen i s small however, lineshape analysis o f the C i s spectrum is p o s s i b l e , e m p l o y i n g components o f FWHM o n l y s l i g h t l y g r e a t e r t h a n 1.4eV and a t b i n d i n g e n e r g i e s a p p r o p r i a t e t o t h e s t r u c t u r a l t y p e s summarized above ((), 1 5 ) . The b i n d i n g e n e r g i e s o f t h e O l s c o r e l e v e l s , a l t h o u g h somewhat l e s s p r e d i c t a b l e , c a n be a p p r o x i m a t e l y g r o u p e d as f o l l o w s : 0-C > i n carbonate

^535.OeV

0-C in acid, ester

> 0-C ^ 0=C i n alcohol, i n ketone ether

^534.3eV

^533.6eV

>

0=C i n acid ester, carbonate ^532.8eV

H o w e v e r , l i n e s h a p e a n a l y s i s o f c o m p l e x O l s band p r o f i l e s c a n o n l y u s u a l l y be a c c o m p l i s h e d a t a c o n f i r m a t o r y r a t h e r t h a n d i a g nostic level. F i g u r e 12 shows t h e C i s s p e c t r a f o r s a m p l e s o f p o l y p h e n y l e n e

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

310

PHOTON,

Figure 12. The C-ls spectra of polypheny lene oxide: (a) sample exposed in a shaded configuration and (b) sample exposed under a Pyrex glass slide (3 months)

(Jul-Aug.)

Figure 13.

(Feb-May)

ELECTRON,

, , 292

A N D ION

PROBES

[ , 288 284 binding energy (eV)

(Feb-Aug.)

Summary of polyphenylene, oxide weathering data in the form of a bar chart ((\J) full; (Zft) Pyrex; fED shaded)

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

18.

DILKS

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o x i d e u s e d i n an i n v e s t i g a t i o n o f t h e n a t u r a l w e a t h e r i n g o f p o l y m e r s u r f a c e s ( 1 6 ) . The b o t t o m s p e c t r u m i s f o r t h e s t a r t i n g m a t e r i a l , ( a ) i s f o r a s a m p l e e x p o s e d f o r 3 months shaded f r o m t h e sun and r a i n and ( b ) i s f o r a sample e x p o s e d u n d e r a 1mm P y r e x glass s l i d e . The w e a t h e r i n g s i t e was i n San J o s e , C a l i f o r n i a . The C i s s p e c t r u m o f t h e s t a r t i n g m a t e r i a l i n F i g u r e 12 shows two m a j o r components a t b i n d i n g e n e r g i e s o f ^ 2 8 5 . 0 and ^ 2 8 6 . 6 e V . These c a n be a s s i g n e d t o c a r b o n atoms n o t a t t a c h e d t o o x y g e n and c a r b o n atoms i n t h e e t h e r l i n k a g e , r e s p e c t i v e l y . A shake-up s a t e l l i t e , shifted *WeV h i g h e r on t h e b i n d i n g e n e r g y s c a l e , i s a l s o a p p a r e n t due t o TT* IT e x c i t a t i o n a c c o m p a n y i n g p h o t o i o n i z a t i o n i n the aromatic s t r u c t u r e . A f t e r 3 months e x p o s u r e i n t h e shaded c o n f i g u r a t i o n t h e w e l l b e t w e e n t h e d i r e c t p h o t o i o n i z a t i o n p e a k s and t h e s h a k e - u p s a t e l l i t e h a s f i l l e d w i t h components due t o C=0 and 0-C=0 t y p e c a r b o n w h i c h have b e e n formed by a t h e r m a l oxidation process. F o r t h e sample w h i c h h a s b e e n e x p o s e d u n d e r a Pyrex g l a s s s l i d e , the c a u s e d a much g r e a t e r d e g r e From t h e r e l a t i v e i n t e n s i t i e s o f t h e C i s components i t c a n be s e e n t h a t ^ 50% o f t h e c a r b o n atoms i n t h e s u r f a c e r e g i o n a r e a t t a c h e d t o o x y g e n , and a l t h o u g h a l o w c o n c e n t r a t i o n o f c a r b o n y l f e a t u r e s i s present, the spectrum corresponds e s s e n t i a l l y to a 1:1:2 r a t i o o f 0 = C - 0 , C - 0 and CH f e a t u r e s . I t w o u l d , t h e r e f o r e , seem t h a t w e a t h e r i n g i n t h i s c o n f i g u r a t i o n h a s r e s u l t e d i n a l m o s t 100% c o n v e r s i o n o f the m e t h y l groups at the s u r f a c e to c a r b o x y l i c a c i d s , w i t h l i t t l e other change. A more c o n v e n i e n t way t o d i s p l a y d a t a s u c h as t h a t shown i n F i g u r e 12 i s i n t h e f o r m o f a b a r c h a r t , i n w h i c h e a c h component i n t h e s p e c t r u m i s r e p r e s e n t e d by a b a r whose h e i g h t c o r r e s p o n d s to the area under the c u r v e . F i g u r e 13 s u m m a r i z e s t h e d a t a f o r p o l y p h e n y l e n e o x i d e , i n t h e form o f b a r c h a r t s , f o r samples e x p o s e d f o r 3 months and 6 months and f o r 1 month i n t h e h e i g h t o f t h e summer. F o r each c h a r t the b a r s r e p r e s e n t , from l e f t t o r i g h t , c a r b o n n o t a t t a c h e d t o o x y g e n ( o f f s c a l e ) , C - 0 , C=0 and 0=C-0. The c o n c l u s i o n s drawn from F i g u r e 12 a r e m a r k e d l y emphas i z e d by t h e c e n t e r c h a r t i n F i g u r e 1 3 . I t i s i n t e r e s t i n g to n o t e , h o w e v e r , t h a t i f a f r e s h sample i s e x p o s e d f o r o n l y 1 month mid-summer t h e C-0 component d e c r e a s e s f o r t h e " P y r e x " and f u l l y exposed samples p r o v i d i n g e v i d e n c e t h a t , under t h e s e c o n d i t i o n s , the e t h e r l i n k a g e i s c l e a v e d . XPS i s a b l e t o answer t h e q u e s t i o n o f how t h e w e a t h e r i n g a t a p o l y m e r s u r f a c e d i f f e r s from t h a t i n t h e b u l k m a t e r i a l and i s a b l e to d e t e c t changes i n the s u r f a c e c h e m i s t r y o f weathered polymers a t a s t a g e where t h e d e g r e e o f b u l k o x i d a t i o n i s s m a l l . M o d i f i c a t i o n s o f p o l y m e r s u r f a c e s by e x p o s u r e t o e l e c t r i c a l p l a s m a s and d i s c h a r g e s have a l s o b e e n s u b j e c t e d t o XPS e x a m i n a t i o n i n s e v e r a l r e c e n t a r t i c l e s ( 4 , 6^, 7). A n example i s the p l a s m a o x i d a t i o n o f p o l y e t h y l e n e , p o l y p r o p y l e n e and p o l y s t y r e n e i n a radiofrequency i n d u c t i v e l y coupled system ( 6 ) . F i g u r e 14 shows t h e C i s and O l s s p e c t r a o f a p o l y e t h y l e n e f i l m after

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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29/

289 287 285 283 28/

538 536 534 532 530

Binding energy (eV)

Figure 14. Core-level spectra of a polyethylene film after various times of exposure to an oxygen plasma

Polyethylene

Polystyrene

20

20

% of

C/s 10-

10—o

-o

40

50

;o

o-

o

o-

50

20

Exposure time (sees.)

Figure 15. Plots of the intensities of the C-ls components vs. t for polyethylene and polystyrene ((* C—O; (U)£=0

(O—C—O); (0)C=0;

(X)£POO)

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

18.

DILKS

XPS Studies of Polymer Surfaces

313

e x p o s u r e t o a pure o x y g e n p l a s m a a t r e d u c e d p r e s s u r e , as a f u n c t i o n o f time o f exposure. Although untreated polyethylene shows o n l y a s i n g l e p e a k , i n t h e C i s r e g i o n a t 2 8 5 . O e V , t h e sample t r e a t e d f o r j u s t 1 s e c . shows a w e l l d e v e l o p e d t a i l t o h i g h e r b i n d i n g e n e r g y and a b r o a d e n v e l o p e a p p e a r s i n t h e O l s r e g i o n . The t a i l i n t h e C i s s p e c t r u m i s a s s o c i a t e d w i t h t h e o x i d a t i o n p r o d u c t s and c a n b e r e s o l v e d i n t o t h r e e components due t o C - 0 , C=0 + O - C - 0 and 0 = C - 0 , from l o w t o h i g h b i n d i n g e n e r g y . Longer e x p o s u r e s t o t h e p l a s m a i n c r e a s e s t h e d e g r e e o f o x i d a t i o n and an a d d i t i o n a l component a p p e a r s i n t h e C i s s p e c t r u m due t o t h e formation o f carbonate s t r u c t u r a l features. By f o l l o w i n g t h e i n t e n s i t i e s o f t h e v a r i o u s components as a f u n c t i o n o f t i m e o f e x p o s u r e t o t h e p l a s m a i t i s p o s s i b l e t o o b t a i n m e c h a n i s t i c and k i n e t i c i n f o r m a t i o n on t h e p r o c e s s e s i n v o l v e d . The O l s s p e c t r a i n F i g u r e 14 a r e n o t so r e a d i l y r e s o l v e d b u t are e n t i r e l y c o n s i s t e n t i term f band p r o f i l d intensity w i t h t h e f i n d i n g s from exposed to the plasma i n c r e a s e s and t h e c e n t r o i d o f t h e O l s e n v e l o p e s h i f t s t o h i g h e r b i n d i n g energy. T h i s s h i f t c a n be a s s o c i a t e d w i t h t h e a p p e a r a n c e o f a component a t ^ 5 3 5 . O e V due t o t h e s i n g l y bonded o x y g e n i n t h e carbonate f u n c t i o n a l i t y . I t i s q u i t e s t r a i g h t f o r w a r d t o m o n i t o r t h e v a r i o u s compone n t s o f t h e C i s s p e c t r a v e r s u s t i m e o f e x p o s u r e t o t h e p l a s m a (6^). F i g u r e 15 e x h i b i t s t h e r e l e v a n t p l o t s f o r p o l y e t h y l e n e and p o l y styrene. The o x i d a t i o n f o r p o l y s t y r e n e i s v e r y much more r a p i d than f o r p o l y e t h y l e n e but both tend to reach t h e i r f i n a l surface c o m p o s i t i o n s a f t e r ^ 16 s e e s i n t h e p l a s m a . The p o l y s y t r e n e a l s o e x h i b i t s a g r e a t e r uptake o f oxygen i n the form o f carbonate and, C=0 a n d / o r O - C - 0 f u n c t i o n a l i t i e s , t h e amounts o f C - 0 and 0=C-0 b e i n g a p p r o x i m a t e l y t h e same f o r t h e two s a m p l e s a f t e r extended exposure times. The g e n e r a l shapes o f t h e c u r v e s i n F i g u r e 1 5 , i n p a r t i c u l a r t h e i n i t i a l peak i n t h e c o n c e n t r a t i o n o f C - 0 and O - C - 0 h a s l e d , when compared o t o t h e r d a t a , t o t h e c o n c l u s i o n t h a t o x i d a t i o n i n the oxygen plasma o c c u r s v i a the i n i t i a l c r o s s l i n k i n g o f t h e p o l y m e r c h a i n s by e i t h e r l i n k a g e s f o l l o w e d by a s u b s e q u e n t f o r m a t i o n o f more c o m p l e x o x y g e n - c o n t a i n i n g f e a t u r e s (6). The a n a l y t i c a l d e p t h p r o f i l i n g f o r t h e s e s y s t e m s ( e . g . t h e p o l y s t y r e n e d a t a i s shown i n F i g u r e 5 ) r e v e a l e d t h a t t h e r e a c t i o n i s e s s e n t i a l l y c o n f i n e d t o t h e topmost m o n o l a y e r o f m a t e r i a l (j>). T h i s i s e n t i r e l y r e a s o n a b l e i n terms o f the plasma c h e m i s t r y s i n c e t h e most p r o m i n e n t r e a c t i v e s p e c i e s i s a t o m i c o x y g e n w h i c h i s e x p e c t e d t o have an e x t r e m e l y s h o r t mean f r e e p a t h i n h y d r o c a r b o n polymers. T h i s s e r v e s as a v e r y good e x a m p l e o f t h e p o w e r f u l n a t u r e o f XPS when a p p l i e d t o t h e s t u d y o f t h e s u r f a c e m o d i f i c a tion of polymers. In a d d i t i o n to the r e l a t i v e l y simple o x y g e n - c o n t a i n i n g s t r u c t u r a l f e a t u r e a l r e a d y d i s c u s s e d , t o be a b l e to o b t a i n a c o m p l e t e p i c t u r e o f an o x i d i z e d p o l y m e r s u r f a c e i t is also

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

314

PHOTON, E L E C T R O N ,

Binding energies (eV)

Compound

0„.

C„(CH)

C (CCj

PhlCH^O-OH

2850

2867

(CH ) CO-OH 3

2850

2866 5341

((CH ) CO}

285 0

286 6

3

3

Figure 16. The C-ls and O-ls binding energies observed in peroxy compounds

A N D ION PROBES

(P
SAMPLING £ DEPTH d/COS0

X(X) 10 15 20 30

TYPICAL MEAN FREE PATHS IN POLYMERS

C

|s

9 5 % SIGNAL (&) 30 45 60 90

(960eV)«l5A

Figure 4. Angularly dependent studies for polymer samples in which spectra are studied as a function of electron take-off angle 6 with respect to the sample surface

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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I n F i g u r e 5 i s shown t h e C and 0 ^ c o r e l e v e l s p e c t r a from t h e a n g u l a r d e p e n d e n t XPS s t u d i e l , XPS? 0 ) , on c o p o l y m e r B c a s t from c h l o r o f o r m . S p e c t r a were r e c o r d e d a t t h r e e d i f f e r e n t angles, 0 = 0 , 4 5 , and 8 0 ° , i n o r d e r t o a c h i e v e effective sampling depths of ^50, ^ 2 5 , and ^ 10 A , r e s p e c t i v e l y . Both the and 0 ^ c o r e l e v e l s p e c t r a c l e a r l y show t h a t e a c h component of the copolymer i s present at a l l sampling depths. Furthermore, from t h e r e l a t i v e i n t e n s i t y r a t i o s o f t h e d e c o n v o l u t e d C ^ peaks a s s o c i a t e d w i t h t h e PS and PEO c o m p o n e n t s , we f i n d t h a t i n p a r t i c u l a r c a s e s , t h e r e i s a c o n c e n t r a t i o n g r a d i e n t i n t h e t o p 50 A o f t h e s u r f a c e and t h a t t h e r e l a t i v e c o n c e n t r a t i o n o f PS and PEO i n c r e a s e s as t h e a i r - s o l i d p o l y m e r i n t e r f a c e i s a p p r o a c h e d . A l t h o u g h t h e f i n d i n g on t h e c o m p o s i t i o n a l v a r i a t i o n b e t w e e n t h e s u r f a c e and t h e b u l k i s i n t e r e s t i n g t h e XPS( 0 ) s t u d i e s r e v e a l a much more i n t e r e s t i n g phenomena i n t h e t o p o g r a p h y . Through the use o f d i f f e r e n c e s i n t h e e l e c t r o n mean f r e e paths(15^) f o r p h o t o e m i t t e d e l e c t r o n s from l

g

g

g

c o r e l e v e l i n t e n s i t y r a t i o s f o r C ^ g / O ^ g a s s o c i a t e d w i t h t h e PEO component a u n i q u e s u r f a c e s t r u c t u r e emerges f o r t h e t o p o g r a p h y , and t h e m o d e l s a r e i l l u s t r a t e d i n F i g u r e 6 . The m o d e l s a - c f o r t h e s u r f a c e t o p o g r a p h y c a n be e l i m i n a t e d from a c a r e f u l e x a m i n a t i o n o f the e x p e r i m e n t a l r e s u l t s . However t h e e x p e r i m e n t a l d a t a s u p p o r t s m o d e l d and f o r t h e s e d i b l o c k c o p o l y m e r s c a s t from c h l o r o f o r m , a m u t u a l s o l v e n t f o r PS and P E O , t h e r e a p p e a r s t o be a t e n d e n c y t o form v e r t i c a l i s o l a t e d domain s t r u c t u r e s at a l l the copolymer c o m p o s i t i o n s s t u d i e d , w i t h a n o n - p l a n a r s u r f a c e o f PS p r o t r u d i n g above t h e PEO d o m a i n . 2) P o l y ( e t h y l e n e o x i d e ) / P o l y ( s t y r e n e ) T r i b l o c k ( P E 0 / P S / P E 0 ) C o p o l y m e r s . A p p l y i n g t h e XPS ( 0 ) t e c h n i q u e t o t h e s e samples c a s t from c h l o r o f o r m r e s u l t s i n t h e s u r f a c e - v s - b u l k c o m p o s i t i o n f o r t h e t r i b l o c k copolymers i l l u s t r a t e d i n F i g u r e 3, a l o n g w i t h the d i b l o c k c o p o l y m e r s d i s c u s s e d p r e v i o u s l y . The s t r i k i n g s i m i l a r i t y i n t h e d a t a f o r t h e d i b l o c k and t r i b l o c k c o p o l y m e r s i n d i c a t e t h a t t h e surface topography i s s i m i l a r i n both cases. The e x p e r i m e n t a l e v i d e n c e i n d i c a t e s t h a t t h e PS and PEO components i n t h e c o p o l y m e r s a r e b o t h e x p o s e d a t t h e s u r f a c e and t h a t t h e y a r e o r g a n i z e d i n t o domains w h i c h a r e t h i c k compared t o t h e XPS s a m p l i n g d e p t h . (3,15) A s l i g h t a n g u l a r dependence i s o b s e r v e d as 0 i s v a r i e d and t h i s a g a i n p o i n t s t o a n o n - p l a n a r s u r f a c e t o p o g r a p h y i n w h i c h PS domains a r e e l e v a t e d s l i g h t l y above t h e PEO d o m a i n s . T h e r e h a v e b e e n numerous s t u d i e s e m p l o y i n g c a l o r i m e t r i c ( 1 9 ) , dynamic m e c h a n i c a l , ( 2 0 ) d i e l e c t r i c , ( 2 1 ) and m o r p h o l o g i c a l ( 2 3 , 2 4 ) techniques to e l u c i d a t e the s o l i d - s t a t e b e h a v i o r o f s t y r e n e - e t h y l ene o x i d e b l o c k c o p o l y m e r s . These measurements h a v e f o c u s e d on t r a n s i t i o n - t e m p e r a t u r e phenomena, and t h e y h a v e p r o v i d e d r e f e r e n c e d a t a on t h e b u l k p r o p e r t i e s o f t h e c o p o l y m e r s . The e v i d e n c e a c c u m u l a t e d t o d a t e i n d i c a t e s t h a t PS and PEO a r e i n c o m p a t i b l e i n t h e b u l k . W h i l e t h i s a p p e a r s t r u e , i n g e n e r a l , one c a n n o t r u l e o u t t h e p o s s i b i l i t y t h a t PS and PEO h a v e some l i m i t e d d e g r e e o f m i s c i b i l i t y i n the c o p o l y m e r s . I t i s a l s o unknown, a t t h i s t i m e , what i n f l u e n c e an i n t e r f a c e ( e . g . , t h e a i r - p o l y m e r i n t e r f a c e ) has

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

19.

THOMAS A N D O ' M A L L E Y

Multicomponent Polymer Systems

327

Figure 5. The C-ls and O-ls core-level spectra for XPS(O) studies on PS/PEO diblock copolymer (B) cast from chloroform. The experimental intensity ratios are corrected for absolute signal from Table II to obtain the molar ratios.

In Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

328

PHOTON,

E L E C T R O N , A N D ION PROBES

on p o l y m e r - p o l y m e r c o m p a t i b i l i t y and w h e t h e r t h e d e g r e e o f p h a s e s e p a r a t i o n i s t h e same o r d i f f e r e n t i n t h e b u l k and i n t h e s u r f a c e . The p r e s e n t s t u d y , i n p a r t , a d d r e s s e s t h e s e i s s u e s by u n i q u e l y f o c u s i n g on t h e s u r f a c e p r o p e r t i e s r a t h e r t h a n on t h e b u l k p r o p e r t i e s o f the copolymers. Our d a t a s u g g e s t t h a t , i n d e e d , t h e r e i s some c o m p a t i b i l i z a t i o n o f PS and P E O . I n t h i s s e c t i o n , we w i l l d i s c u s s t h e i n t e r e s t i n g r e s u l t s on the i /°i intensity ratios for t h e PEO component and the C ^ ( TT *•«- TT ) / C - ^ i n t e n s i t y r a t i o s f o r t h e PS component i n t h e t r i b l o c k copolymers. T a b l e I I I c o n t a i n s the measured i n t e n s i t y r a t i o s f o r t h e two homopolymers and t h e t h r e e t r i b l o c k c o p o l y m e r s . The t r i b l o c k s d e v i a t e from t h e homopolymers i n two w a y s : First, they have u n u s u a l l y h i g h ^ / 0 - ^ r a t i o s f o r t h e PEO component compared t o t h e PEO h o m o p o l y m e r , and s e c o n d , t h e y have u n u s u a l l y l o w C ( TT *